We have seen a balanced bridge circuit in the basic circuit of NIC...
https://www.researchgate.net/post/What_is_the_basic_idea_behind_the_negative_impedance_converter_How_is_it_implemented_How_does_it_operate_What_does_the_op-amp_do_in_this_circuit/3
... and its various applications - current mirror, Wien oscillator, Deboo integrator...
https://www.researchgate.net/post/Can_a_negative_impedance_converter_NIC_act_as_a_current_mirror_If_so_is_it_a_perfect_circuit_What_kind_of_NIC_do_we_use-INIC_or_VNIC/4
We have shared the belief that the balanced bridge circuit is omnipresent and can be seen in many applications. But entering, with a children curiosity:) more deeper and deeper into the essence of things, we finally arrived at the most fundamental questions, "What is a balanced bridge circuit? What are its unique properties? What and where is the 'bridge' there? What is its role?"
Thinking on these primary questions, I finally figured out that the simple basic idea of a balanced bridge circuit is to connect two equal voltage sources by a third, passive element. The voltage sources are grounded while the coupling element is "floating". So, the most elementary bridge circuit consists, all in all, of three elements - two grounded sources and one floating element. This element, "stretched" between the sources, is actually the "bridge" that connects them. Figuratively speaking, the floating element is the "bridge over the river"... and the two sources are the "banks of the river":)
In circuitry, it is inconvenient for us to use multiple voltage sources. So, we replace (implement) them by multiple voltage dividers (two resistors in series) supplied by only one single source. This yields the famous 4-resistor bridge circuit where a fifth element is connected between two equipotential points. The most interesting questions here are about the properties of this "bridge element".
It is placed at very special conditions - both the voltage across and the current through it are zero... but it can have any resistance - from zero to infinity. I.e., in the case of a well-balanced bridge circuit, whatever we connect between the two points (even to short or leave them unconnected), nothing will change... Or, if we connect some element... and then balance the circuit... we can remove the element... and nothing will happen...
After I directed the discussion towards more unusual perspectives on this legendary circuit, I propose to continue further below in this direction...
Dear Cyril,
Dear respected colleagues,
I would like at first to thank Cyril for dedicating the previous question to me. I am very indebted to him.
As for the the bridge circuits originated by Wheatstone as a DC bridge and extended by many scientists for AC as AC bridges are thought for measuring the circuit elements , R,Land C. The measurement of the elements are based on the so called null measurement concept where the voltage detector ic the circuit just is used to indicate the zero voltage in the bridge rather calibrated to indicate the the value of the element. This is the case of the deflection type measurement where the measuring instrument is calibrated to indicate directly the value of the element. The null type measurement using the bridges and the null detector is characterized by more accuracy than the deflection type using the voltage over current ratio,
The bridge method is called also the comparison method where eg the value of the unknown element say Rx is determine by other known resistance.
As foe the classical bridge circuit it is described by Cyril.Taking whetstone as a model it consists of two half bridges fed by a voltage source and a detector connected at the the common point of very half. The detector represents the bridge between the two halves as Cyril hinted before. So, there are four resistors in the different branches of the bridge. Say R1, R2 1N THE LEFT SIDE AND R3AND R4 IN THE RIGHT SIDE.
The condition of balance where he detector voltage is zero, and assuming R1=R3,
results in R2=R4.
Following this condition:
The voltage on R2= the voltage on R4, and
The current downward in R2= the current flowing downward in R4.
Now let us substitute R2 by a constant voltage source Vi with the positive polarity connected to the common point with R1 and the detector.
Now the current is still flowing downward in the voltage source in the apposite of its normal current flow. This is the effect of the negative resistance. That is the bridge at balance seen from the the common point between the detector and R1 is equivalent to a negative resistance with assuming that this port sinks the current at positive input voltage.
To realize this circuit and get an automatic near balance condition and not exactly a balance condition the detector is merely substituted by an op amp whose output substitutes the feed source of the bridge. This configuration results in the negative impedance converter.
So using the concepts of he bridge one can easily sees then negative impedance effect.
One has to stress that in the negative impedance convener circuit with op amp the balance condition is approximate rather than exact, But if the input difference voltage is smaller the condition will be better realized.
My best wishes
The first topic of discussion can be, "How do we balance the bridge circuit?"
In my opinion, the main property of the balanced bridge circuit is the equality between two voltages. We can achieve it by varying the one, other or both voltages.
The conventional 19th century way to do it is by changing the resistance of some branch... the one, the other or both belonging to the same leg (in the opposite directions). This means to change the transfer ratios... accordingly, the output voltages of the two voltage dividers (half-bridges).
Some more exotic ways can be by inserting voltage sources in the branches (in series with the resistors)... as in the case of a NIC... or to connect current sources in parallel... In these cases, the bridge legs (halves) act as summers...
The more unusual way can be to change the supply voltage... as in the case of a NIC... To reach the equilibrium, the two voltages should converge (approach to each other)... This means the two transfer ratios should be different and be in a given proportion...
If they are equal, the circuit will be always balanced since, when varying the supply voltage, the two partial voltages will simultaneously vary with the same rate and direction... they will act as common-mode signals...
Cyril - for my opinion, at first we should strictly distinguish between the two basic alternatives: (A) Passive bridges and (B) Self-adjusting active bridges (opamp-based) where the error voltage directly controls one of the bridge supply voltages.
Maybe we should consider A as building components of B? I.e., to consider the op-amp bridge circuit consisting of a passive bridge circuit + op-amp?
In my opinion, the op-amp is nothing to do with the pure "bridge idea"... it is only one possible implementation... So, I prefer to stay in 19th century and manually change the supply voltage with the purpose to balance the bridge. It would be very simple and easy for us... and we will not be distracted by non-essential in the case elements such as the more sophisticated op-amp... while actually the humble passive bridge is the most important thing here. As far as I know, in the classic measurement courses, they continue using such "man-balanced bridges"...
The op-amp will only mask the pure idea and make it incomprehensible. Why? Because to understand what actually an operational amplifier does, we ourselves must be put in its place (empathy).
The op-amp is needed for people who do not understand the pure idea and need somehow to disguise it (involving all kinds of details like frequency and gain restrictions, voltage offsets, etc...)
So, I suggest to think of this arrangement as of a manually-controlled servo system where we continously keep the balance...
Now I would like to continue "philosophizing" on the nature of the bridge circuit...
In my opinion, it is a "bridge circuit" because of two reasons: first, since the two voltage sources are grounded; second, simply since we want to see a bridge inside it:)...
If the "bridge" circuit is floating... and we draw it in a different way (e.g., as a loop - see the attached picture)... it will be no bridge circuit... it will be simply a circuit consisting of three elements in series - two identical voltage sources compensating each other, and a load...
So, I say it again: the naked truth is that the bridge circuit consists of three elements in series - two voltage sources and one passive element.
An interesting question could be whether we can swap some elements, e.g. to ground the one voltage source and the passive element... and to "stretch" the other voltage source as a "bridge"?
Anyway, we have two points with the same voltage relative to ground, connected by a "bridge" element ("jumper")...
Looking from the side of the one point (the active terminal of the one voltage source) at the other point (source), we see... nothing... This is the well known bootstrapping effect...
https://en.wikipedia.org/wiki/Miller_theorem#infinite
If I look at it, knowing some bridges, in my opinion, a 'bridge' is a 4-element construction, eg with 4 resistors, with 2 arms, each consisting of 2 resistors in series.
For the bridge, the input is the top-bottom source, the output are the left-right midpoints of the 2 arms.
The 'oldest' bridge is the resistor bridge, you also have other bridges:
- With resistors and capacitors for op-amp oscillator circuits
- With diodes for rectifiers. Then , the diodes rectify from the AC top-bottom to DC left-right.
- With diodes or MOS transistors for RF mixers. For an RF mixer, the diodes are connected different than for a rectifier: They are connected in a ring.
- Again, with diodes, for a sample/hold circuit.
- With power transistors (IGBT, MOS, Bipolar, thyristor) for inverters.
(Some inverters have 3 arms and 3 outputs to steer eg triphase motors or the triphase grid.)
For the sample and hold, I am not sure, but to my knowledge the other circuits are all called 'bridges'. philosophycally, I dont know what to make from it.
Greetings,
Henri
Cyril - I am very sorry, but I have some problems to verify this sentence:
"So, I say it again: the naked truth is that the bridge circuit consists of three elements in series - two voltage sources and one passive element."
Please, can you explain this based on a simple example?
Lutz, my idea is that the name "bridge circuit" has originated from the fact that a floating element (galvanometer, voltmeter, op-amp input...) is "stretched" like a bridge between the two sides acting as grounded voltage sources. Shortly, I suppose "bridge circuit" means that two voltage sources are connected with a "bridge element".
So, in total, there are three elements connected in a form of a loop - two grounded voltage sources and a floating element between them. Actually, they are connected in series... and since the two voltages are equal and opposite, no current flows through the loop... and there is no voltage across the voltage indicator...
An example of this arrangement can be any op-amp circuit with serial negative feedback (e.g., a non-inverting amplifier)... where (a part of) the output voltage is connected in series to the input voltage...
But really it is a pretty broad interpretation of the name "bridge circuit"... and it may be challenged...
Cyril - with reference to your above answer: Does this mean that you consider the midpoints of both bridge arms (common node of R1, R2 and R3, R4) as voltage sources?
Introducing the concept of a bridge in this context seems to me a little forced.
I prefer to use the theory of feedback.
Cyril - OK, now I understand your position. However, I am asking myself if such a view could be helpful?
I prefer to use the theory of feedback.
Josef - yes, I agree with you because of the following reason: The NIC block tends to static instability (one type is open-loop stable and the other one is short-circuit stable). This property must always be taken into consideration. If we treat the NIC as a bridge circuit only this important application restriction is "out of sight".
Dear colleagues,
The theory is a theory... but here I want to disclose, in an intuitive and human-friendly way, the basic concepts on which the bridge circuit is based. According to me, it is not appropriate at this level to introduce another basic concept as negative feedback because it will distract and sent us elsewhere...
As I have said before, we can consider the bridge circuit as a component of a negative feedback system - a feedback network with a single-ended input and a differential output... that is connected between the op-amp output and its differential input. Here we discuss the unique properties of this component...
Cyril, I must admit that - for a moment (my last contribution) - I forgot that in this thread you want to discuss the "idea of a balanced bridge circuit" in general - and not only the special case of a "NIC-bridge" .
To be honest Lutz, I have started from this initial "NIC point"... but later I was tempted to deal only with the mere bridge circuit ... like an even bigger challenge:)
Because, you know and will second me, it is easy to "understand" (know) complicated things in this world... but it is much more difficult to understand really the essence of utmost simple things:)
Perhaps our discussions confirm in the best way this maxim...
Well, I will try to return the discussion in a more constructive direction by putting some more interesting observations for consideration...
As Henri has noted above, "the 'oldest' bridge is the resistor bridge" (in the attached picture below). Let's try to describe, by means of simple human words, what this 4-resistor circuit is... and what it actually does.
What it is... It consists of two "sub-circuits" - the voltage dividers R1-R2 and R3-R4. Their single-ended inputs are joined and driven by a common grounded voltage source V. The floating differential circuit output is taken between these single-ended outputs.
What it does... The two voltage dividers split and scale (attenuate) the common grounded voltage V into two grounded "sub-voltages" VR2 and VR4. Then these partial output voltages are subtracted in a "serial manner", according to KVL.
So, this is a sort of an "attenuating-subtracting circuit"...
Let's now see what are the interesting moments in this arrangement...
The first interesting thing is the paradox that we split a quantity into two parts and then subtract these two parts from one another. Thus, in the case of a well balanced bridge circuit, this leads to a total "self-destruction" of the input quantity. What can be the sense of this?
In the case of a not so well balanced bridge circuit, the output quantity is a small difference between two big input quantities. Like above, we may ask ourselves, "What can be the sense of this?"
In my opinion, a useful application of this odd idea would be obtaining a very large attenuation ratio by using reasonably small proportions between the resistances of the pairs R1-R2 and R3-R4...
Another benefit of this "attenuating-subtracting configuration" can be its ability to invert the output quantity (I have mentioned it before). We can obtain various transfer ratios depending on the proportion between the two divider's ratios K1 and K2 - positive (K1 > K2), zero (K1 = K2) and negative (K1 < K2).
But the fact that the output quantity is a small difference between two big input quantities may cause a significant error... e.g., because of temperature variations...
Dear Cyril,
Dear respected colleagues,
I would like at first to thank Cyril for dedicating the previous question to me. I am very indebted to him.
As for the the bridge circuits originated by Wheatstone as a DC bridge and extended by many scientists for AC as AC bridges are thought for measuring the circuit elements , R,Land C. The measurement of the elements are based on the so called null measurement concept where the voltage detector ic the circuit just is used to indicate the zero voltage in the bridge rather calibrated to indicate the the value of the element. This is the case of the deflection type measurement where the measuring instrument is calibrated to indicate directly the value of the element. The null type measurement using the bridges and the null detector is characterized by more accuracy than the deflection type using the voltage over current ratio,
The bridge method is called also the comparison method where eg the value of the unknown element say Rx is determine by other known resistance.
As foe the classical bridge circuit it is described by Cyril.Taking whetstone as a model it consists of two half bridges fed by a voltage source and a detector connected at the the common point of very half. The detector represents the bridge between the two halves as Cyril hinted before. So, there are four resistors in the different branches of the bridge. Say R1, R2 1N THE LEFT SIDE AND R3AND R4 IN THE RIGHT SIDE.
The condition of balance where he detector voltage is zero, and assuming R1=R3,
results in R2=R4.
Following this condition:
The voltage on R2= the voltage on R4, and
The current downward in R2= the current flowing downward in R4.
Now let us substitute R2 by a constant voltage source Vi with the positive polarity connected to the common point with R1 and the detector.
Now the current is still flowing downward in the voltage source in the apposite of its normal current flow. This is the effect of the negative resistance. That is the bridge at balance seen from the the common point between the detector and R1 is equivalent to a negative resistance with assuming that this port sinks the current at positive input voltage.
To realize this circuit and get an automatic near balance condition and not exactly a balance condition the detector is merely substituted by an op amp whose output substitutes the feed source of the bridge. This configuration results in the negative impedance converter.
So using the concepts of he bridge one can easily sees then negative impedance effect.
One has to stress that in the negative impedance convener circuit with op amp the balance condition is approximate rather than exact, But if the input difference voltage is smaller the condition will be better realized.
My best wishes
Dear Abdelhalim,
Thanks for the comprehensive explanations about the remarkable bridge circuit properties. Of course, we know very well that discussing the old resistive bridge, we mean all its nowadays applications (enumerated by Henri before)... where all kinds of elements (passive/active, linear/nonlinear, etc.) are included in the bridge arms...
As you can see, there are many equal quantities in the balanced bridge circuit, especially in the NIC applications. First, of course, the total voltages across the two bridge legs are equal to each other and to the supply voltage V. Then, the partial voltage drops across the opposite resistors are equal (VR1 = VR3 and VR2 = VR4). As we have chosen R1 = R3, the currents flowing through the two legs (and the resistors) are equal. Since the voltages across and currents through R2 and R4 are equal, then these resistances are equal as well...
In the bridge configuration, we can apply the KVL in two loops - the upper (R1 - R3 - null indicator) and lower (R2 - R4 - null indicator).
I have established that in the NIC bridge application, it is more convinient to apply the KVL in the upper loop (left R - right R3 - op-amp input)... reasoning as follows:
For example, if an input source is connected at the inverting input, it sinks current from the op-amp output through the right resistor R ... and a voltage drop VR = I*R appears across this resistor. The op-amp reacts to this intervention by passing the same current through the left resistor R and the external load... thus producing the same voltage drop VR = I*R across this resistor. As a result, the difference between the two voltage drops, applied to the op-amp differential input, is almost zero...
So we consider only the upper loop without mind what is included in the lower loop... we have not even portrayed it. This approach is extremely useful when considering the current mirror applications of the NIC.
Dear Cyril Mechkov
Its my pleasure to join this discussion.Your elaborate explanation gives insight of few unknown facts like when the bridge balanced it doesn't matter whether there is an open/short circuit in the galvanometer branch .Apart from conventional method of analyzing ohms law with voltage source and ground,balance/unbalance bridge offers an opportunity to visualize potential difference and current are in direct proportion in the galvanometer branch. Here we find better opportunity to explain potential difference with a floating ground.
with regards
vasanth
To make the discussion funnier... as it should be when discussing such an old and eternal circuit in an intuitive way... here is how my students implemented and investigated it in 2008...
https://en.wikibooks.org/wiki/Circuit_Idea/Group_68a#Wheatstone_bridge
Other questions (with reference to Cyril´s main title):
1) What is this thing called "bridge"?
2) Is the main purpose of a bridge to be balanced (zero voltage difference) ?
3) Are there intentionally unbalanced bridge circuits ?
4) Can a classical common-emitter stage (with base voltage divider) also seen as a bridge (unbalanced)?
At last the discussion began to come alive!
Lutz, thanks for the interesting questions. And I was just about to ask the question whether a long-tailed pair can be considered as a bridge...
Here are my suggestions for possible answers to your questions:
"Bridge circuit" is the whole circuit including the "bridge element".
Cyril - here are my comments:
0) Long-tailed pair: Very intersting aspect. I think, it was DERIVED from a bridge:
Left branch: Common collector stage; right branch: Common base stage.
For symmetrical dimensioning, both emitter nodes are at the same potential (balanced bridge) and the voltage across the bridge element is zero. Hence, both nodes may be connected.
1) Do you mean that the V-meter (which indicates balance or non-balance) forms the bridge? I rather think that (in the simplest case) the resistor branches R1-R2 and R3-R4, respectively, form the device we call "bridge".
2) I rather would say: .....In these applications it may be included in a negative feedback system ...and may be act as...
3) OK
4) I think, to be compared with a bridge, the righ arm should be able to produce a voltage (which is to be compared with the voltage of the left arm.) I doubt if we can say that the transistor produces a voltage at the (open) base node (without the help of the left arm).
Dear Lutz,
You have encouraged me to ask the next question about more possible applications of the bridge configuration...
https://www.researchgate.net/post/Can_we_present_the_long-tailed_pair_as_a_bridge_circuit
Regarding the role of the "V-meter (which indicates balance or non-balance)" I think the following:
From one side, it seems, as you have noted, that it is an external element - the load connected at the output of the circuit. So, from this viewpoint, it does not belong to the bridge.
From the other side, if we detach it from the circuit, what will be the "bridge"... the element which resembles a real bridge?
Now I would like to draw your attention at the implementation by two potentiometers...
It is an interesting fact here that, in the case of a high-resistance load (bridge element), the branch resistances do not change... and we only contact with different points of them...
So, even a more interesting fact is that the currents in the two arms do not change when moving the pot sliders... since the resistances have not changed...
And the most interesting fact is that the voltages also do not change... since they pre-exist!
Expressing this provocative statement, I mean the voltage potentials along the resistive films of the potentiometers. They exist... and we only select a pair of two voltages from the two pots...
So, it turns out this circuit is quite boring since nothing changes inside it:)
Here is an illustration of my thesis above that nothing (neither any resistance nor current nor voltage) changes when moving the potentiometer's slider...
https://en.wikibooks.org/wiki/Circuit_Idea/Walking_along_the_Resistive_Film#Closing_the_circuit
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