A subcircuit is required representing an equivalent circuit of an operational amplifier and a related symbol with the following pins / ports:
- non-inverting input
- inverting input
- output
The main function of an operational amplifier (Op-Amp) is to amplify the input signal, or the difference of two input voltages. To amplify an input signal you need some active elements (components), such bipolar or unipolar transistors (BJTs, FETs, MOSFETs). You can’t make an Op-Amp with only passive elements (components). The ever-popular 741 monolithic Op-Amp, designed by Dave Fullagar in 1967, and fabricated at Fairchild, as μA741, in 1968, consists of 24 transistors, 11 resistors, and 1 capacitor.
I think that there are an equivalent circuits that can be used for op-amp , but it will not give you the same response , may be you can find it in the datasheet or in reference book that explain the characteristics of the op.amp.
Op-Amps ideally have infinite input impedance and gain with zero output impedance, while being arithmetically perfect at all frequencies. This is beyond passives in so many ways. Constructed of discrete elements, a fair and functional device could be very simple: two transistors, and a push-pull driver of two more transistors, with a few passives. JFETs or IGFETs are nice for inputs since the impedance is high with a current near 0. FETs are nice for the drive as well, but bipolars are good too.
Dear Mahmoud Ibrahim Darwish, Bjorn Burton has answered very well. If you want further details you can consult him.
Hello Mahmoud,
You can use BJTs and few biasing resistors to start with. First stage would be an emitter-coupled differential amplifier, ideally followed by another CE stage and finally a push-pull using pnp-npn combination would do. This will have the (INV), (NINV) and the (OUTPUT) pins. But keep it in mind that the performance cannot be compared with the commercially available op-amp chips. Nevertheless, it is possible and you can construct and use one such op-amp using discrete devices. I have myself constructed an op-amp that operates at +300 / -300V supply and delivers 500Vp-p output for a 1kHz, 10Vp-p sinewave input.
Best Luck!
normally, the idea of using passive components is to have low power consumption. Nowdays, we can find amplifier with nA consumption, like MAX4036. But, if you want only elevate the voltage you can use a multiplier voltage (diodes and capacitor).
I agree with Salvatore, you cannot create an op-amp using only passive devices. You will need the aid of active devices.
Now, if your concern is designing an op-amp using "discrete" devices, then Bjorn has gave the answer for you.
Assuming you want this for simulation in Spice, the following looks like a good source of op-amp circuit models, starting from a simple, idealized model:
http://www.ecircuitcenter.com/OpModels/OpampModels.htm
It seems that you need to simulate an op-amp instead to design it. If this assumption is true, please pay attention to the response given by David. You have to use a voltage controlled voltage source, available at all Spice or Spice-like simulators.
The main function of an operational amplifier (Op-Amp) is to amplify the input signal, or the difference of two input voltages. To amplify an input signal you need some active elements (components), such bipolar or unipolar transistors (BJTs, FETs, MOSFETs). You can’t make an Op-Amp with only passive elements (components). The ever-popular 741 monolithic Op-Amp, designed by Dave Fullagar in 1967, and fabricated at Fairchild, as μA741, in 1968, consists of 24 transistors, 11 resistors, and 1 capacitor.
That's a great example by Nebi Caka. The 741 was the all-tool of the '70s! Going back, there were some Philbrick GAP/Rs in a surplus shop in Los Angeles that I was temped to buy. The schematic is attached (from http://www.analog.com/library/analogdialogue/archives/39-05/web_chh_final.pdf). I also found a nice article by the late Bob Pease on the P85A. (http://www.philbrickarchive.org/notes_on_the_design_of_the_p85.htm)
It may be possible to simulate (or build) a form of GAP/R using four Supertex LN150's (N-Channel Depletion parts). They have a SPICE model. The "8355037" seems to be a thyrite resistor once made by G.E.
Interesting discussion so far!
Bjorn has added nice piece of information. Yes, vacuum tubes could only operated at higher voltages, and as in the example, vacuum tube beauties were used to construct an op-amp. The triode itself was an analog of transistor in those glorious days to amplify signals.
Nevertheless, the op-amp that I designed and was used in field trails has all solid-state (transistors) devices to operate at 700V supply.
Thanks for greater insight into this topic!
You can't! As Nebi Caka well said, you cannot amplify without active elements.
Hmm, step-up transformer?
Voltage amplification.
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Yes, I know. Not the point. :)
Hello Neil,
Can we really call a transformer (such as step-up) as an amplifier?
I invite vies and comments from our other friends too!
Run a transformer through saturation and you have a magnetic amplifier. Even a push-pull configuration is possible, but at this point it's certainly not passive---and still a long way from an op-amp.
Hello Neil,
Can we really call a transformer (such as step-up) as an amplifier?
I invite views and comments from our other friends too!
Dear Prasanna Waichal, Good you have opened the discussion further. Are we talking about the amplification functionality or OAMP. As far as the functionality is concerned we can achieve even with potential divider. "From the functionality point of view, yes we may call transformer as amplifier".
The transformer is an electrical (generally a four-terminal) device that uses magnetically coupled coils to transfer energy from one circuit to another. The coil on the input side, directly connected to the voltage source, is called the primary winding. The coil on the output side, connected to the load, is called the secondary winding.
In an ideal (lossless) transformer the output power is equal to input power:
Pout ≡ Ps ≡ P2 = V2xI2 = Pin ≡ Pp ≡ P1 = V1xI1
giving equation of the ideal transformer:
V2/V1 = N2/N1 = I1/I2
or:
V2 = (N2/N1)V1; I2 = (N1/N2)I1
where: N1 is the number of turns of primary winding, and N2 is the number of turns of secondary winding.
As we can see from the above equation: For a given increase in output voltage V2, there is an equivalent decrease in the output current I2.
So, the transformer can not amplify (step up) an AC input voltage without reducing (stepping down) output current, which means it can not amplify (increase) the power of a signal.
An amplifier, on the other hand, is an electronic device that can amplify (increase) both current and voltage at the same time, meaning it increases the power of a signal by taking energy from a power supply.
The normal transformer is different than the one used in magnetic amplifiers. In fact magnetic amplifiers are saturable core reactors that resemble a transformer. The main difference is that the core is made up of special material having sharp (ideally rectangular) B-H curve (hysteresis loop) while the one used for common transformers are having a slowed loop.
And for a push-pull configuration again the active devices like tubes or transistors come into picture.
Apologies, Mahmoud Ibrahim Darwish
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Seems to have become widely side-tracked.
I simply tried to answer something that seemed to be leading to some other place.
Again, sorry.
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I realise that you asked a question, probably not aiming to start a broad discussion of the definition of amplifier, the characteristics of transformers, linear & non-linear.
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Did you get what you wanted?
From your question it was not really obvious what you wanted.
Did you want an equivalent circuit to an opamp in discrete components?
A SPICE model?
Not possible,since to amplify the any signal you need a passive element. The parametric amplifiers though use only passive elements but can not work as operational or differential amplifier.
Mahmoud, maybe your question should be, "Can someone advise on how to design an op-amp using descrete elements?"
Passive component cannot make Op Amp,since they can't amplify. Passive component will only consume your signal/voltage. You can't bye-pass Op amp(Active components)
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