The best way to show the basic idea of a circuit is to build it step-by-step. I do this every year with my students at the beginning of the laboratory exercise about latches in the laboratory of digital electronics. You can see how in the movie below and also in this Wikibooks story written by my students:
http://en.wikibooks.org/wiki/Circuit_Idea/Group_67b#Lab_4:_Endowing_circuits_with_memory
Here is the building "scenario":
1. To make a latch (flip-flop, memory cell, 1-bit RAM...), we need only a non-inverting amplifier - we just connect its output to the input... and it begins "memorizing"... keeping by itself at the "top" (+Vcc) or the "bottom" (ground). BTW this clever idea was proposed in 18th century by Baron Münchhausen... so that he can be considered as the inventor of the most elementary RAM cell:)
2. But, for some reasons (?), in electronics there are only inverting amplifying stages - a common-emitter amplifier, in this case. So, to make a non-inverting stage, we connect (by the yellow wire, in the picture below) in succession (cascaded) two inverting stages, and then connect (by the black wire) the output of this non-inverting amplifier to its input. Actually the two stages are connected in a loop... but for some reasons (maybe to hide the idea:) they draw this configuration as two cross-coupled stages. We should connect the base resistors to extend the small (0.7 V) input range of the the bipolar transistor up to Vcc (+12 V). If we do not connect them, the forward-biased base-emitter junction (like a diode) will fix the collector voltage of the other transistor at +0.7 V, and the LEDs will ever light.
3. We can drive this latch in various ways (as you can see in the movie):
- The most correct of them is to connect the one end of the (red) wire to the ground and to touch the base of the turned-on transistor with the other end, or to touch the collector (the body, if it is made metallic) of the turned-off transistor.
- An incorrect way is to connect the red wire to Vcc (as shown in the picture), and to touch the collector of the turn-on transistor... but it is interesting that it still survives:). It is extremely interesting to see why since exactly in this "incorrect" way the memory cells of today's modern RAMs are controlled...
- And the most incorrect way is to touch the red wire directly to the base of the cut-off transistor (since you apply +12 V across the base-emitter junction that tries to fix it at 0.7 V). The result - more likely, either the transistor or the power supply will be damaged; that is why (to limit the current) we have connected the base resistors. But in the circuit shown in the picture and the movie, the transistors still "survived" when my students touched their bases by +12 V... it is interesting to see why...
https://drive.google.com/open?id=0B45uRPpHPD9hVERNa1pmNl9SOG8
Dear Cyril,
The electronic memory is a very useful device. It is a basic building block in digital computers. The introduction of the electronic memory in the digital communication systems resulted in many fundamental enhancements in such systems and consequentially many new services. So, the study of memory deserve great attention.
The electronic memory is binary such that one memory cell stores one bit, either a 0 bit or one bit. First of all, how can these bits physically represented. According to the TTL logic the zero is represented by a zero voltage and the one is represented by certain specific voltage according to the power supply voltage VDD.
Now let construct a memory cell that can store either one or zero according to the value of the input data.
Since the inverter is the basic building block of the logic, we will use it to build the memory cell. Since the inverter inverts its input logic value, it is not able perform as a memory and also if the input is removed the output will be changed. The solution is to cascade to inverters in cascade which results in Input logic= output logic. But when the input is removed the output could change violating the function of the memory which must retain the written data after the writing process.
In order to achieve the retention the circuit output must be returned to the input for self feeding and sustainability.
The circuit in its last configuration is the bistable flip fop circuit The two states are the set state where the output is high and the reset state where the output is low.
It is the bistability which makes this circuit cats as a memory cell. The change between the two states is affected by the set reset inputs which are in form of limited duration pulses.
There are other type of the electronic memory which is very simple. it is the dynamic RAM consisted of an address transistor and a capacitor. It is so that the information is stored in the capacitor in form of a charge. The zero is Zero charge and The one is a positive charge of sufficient value.
The DRAM is more common that than the SRAM made of S-R flip flops. However they need to be refreshed periodically to retain the data on them.
The re are types of memories other than the electronic memory as the magnetic memory which are used as mass storage in computers.
Best wishes
Abdelhalim, thank you that you dare to open this discussion six months after asking this question. I accept your detailed explanations but I just want to say that my idea was to draw attention to some paradoxical moments of the memory cell control.
The main paradox of this control is that we connect a voltage source to another voltage source what, from the conventional electrical viewpoint, is something very bad and strictly forbidden (imagine this is a "short connection"!!!)... but still it is widely used in computers. The transistor "survives" since the flip-flop instantly reacts to the student intervention by switching to the other state; the transistor turns off and so protects itself (as an automatic fuse).
My final story where my students touch the red wire (+12 V) directly to the base of the cut-off transistor (apply +12 V across the base-emitter junction is a "teaser":) Now really the transistor should "die"... but it still "survives" since the power supply (12 V battery from a remote control) is too weak - its voltage drops when they connect it to base-emitter junction... and so it "comes to life":)
I have revealed the evolution of this powerful idea in the presentation below by building the memory cell step-by-step (from the 18th century Baron Münchhausen bootstrapping:) to the present 6-transistor memory cell).
(Just to squeeze one more note: I allow myself to joke here because scientists have well-developed sense of humor and should not be offended by such fictional stories; rather such joking should provoke their creativity.)
Dear Cyril,
The role of the circuit designer is to grantee the safe operation of the devices in the circuit under the worst case stress conditions or the worst loading conditions.
The transistor as a circuit element has voltage, current and power ratings at which it can be operated safely without burning or damage. So, one of the ratings is the maximum base current under static and pule stress surge condition. The latter is the fuse effect in which the base can withstand much higher current than the static base current.
To change the state of the transistor a control base current must be conveyed to its base terminal. The best way is to apply the controlling voltage through a base resistance as you said to limit the current to the safe base current. In your circuit it is the point of the collector of the other transistor. If you apply directly the voltage at base of the transistor to be controlled and the transistor does not get damaged, then it is so that the source itself is not an ideal voltage source. As You said it is a weak voltage source. If it is strong it will surely damage the transistor, no way, it will not survive. You can observe the current pulse during the touch and see how strong the current pulse to verify that it did not exceed the safe values given in the data sheet.
I would propose that you use a strong power supply to carry out the experiment. It could not survive long pulses directly at its base.
It is practically so that the flip flop is controlled by short pulses that switches the transistor as fast as possible and preserves the transistor in safe operation and save the power lost in the circuit.
I hope this covers some points raised in your question.
Best wishes.
Yes Abdelhalim, if we connect a voltage source directly to the transistor base, it should be not ideal. But if we connect it to the collector, it can be an ideal voltage source since the transistor instantly switches off and does not manage to burn.
Best regards,
Cyril
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