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A Replacement For The Op-Amp

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Oct 2017 at 8:56am
Because of the work by Butler on the feedback cascode with emitter follower (earlier post) we have a chance of producing a high gain discrete transistor stage.

If we take an 18V power supply and bias a single transistor for symmetrical output it will have a collector voltage of 9V, and from 40 x Vc we find it will give us a gain of 360. What we need is at least a gain of 1000 which is 60dB so we can apply 40dB of that as negative feedback to obtain low distortion for a gain of 10 (20dB) amplifier (40dB suggested by Harold Black of Bell Labs in 1927 which still holds true).

Butler would not have had the luxury of simulation we have since his work, and as it is extremely important to have high frequency stability especially in this RF dominated world of today, we find his circuit needs much improvement.

The following simulator circuit was the result of many hours of empirical juggling:

Inverting Discrete Opamp

It features the current doubling emitter follower current source (Q4 - or should that be 'current sink'?) suggested by Self. This not only lowers its output impedance but makes its output more linear - it is not relying on an emitter resistor to pull the voltage down on negative excursions. This linearity improves the performance. You will also notice several small value capacitors and these are needed for stability as the dominant pole doesn't dominate sufficiently - this being a problem of the cascode as far as this particular gain goes.

And this is the simulation result for bandwidth and phase:

Inverting Discrete Opamp Open-Loop Bandwidth

As expected, and it's the same with op-amps, the frequency response is very limited, but we have around 75dB (x5600) gain. With negative feedback of 40dB or more we can expect a much wider and respectable frequency response. But what we are looking for right now is bandwidth with stability, and we can see that we have a 0dB bandwidth of 7.5MHz and a phase margin of 81 degrees. We also have a gain margin of better than 20dB (factor of 10). This is the gain at -180 degrees, which is 360 degrees in an inverting amplifier like this.

However, unlike the op-amp, if we apply negative feedback by selecting a ratio for Rf and Rin we will find that stability is upset, and there will be a spike in the LF response.

Unless we use the same techniques as used in an op-amp it will always be like this. But we want to try and get away from the problems we think the op-amp dishes up, so we will therefore find that each stage we make using this (and similar discrete circuits) for a particular purpose, will have to be adjusted or tuned to behave for the gain we want - as they always were.

However, it makes a good starting point from which a number of desirable gain stages can be made.

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 06 Feb 2018 at 8:06am
This next stage is the classic 2 transistor multiple feedback design using an extra transistor for a low impedance output.

It cannot be considered as a discrete op-amp but it is a great fall back for where a larger output swing is required than an op-amp which is limited by its supply voltage.

Here it is working on a 36V supply but by adjusting values it can operate at much higher voltages. The transistors would have to be suitably rated.

Classic 2 stage discrete amplifier

Variations on this configuration have featured in numerous audio products since the 1960s and the sound is often far more natural than can be obtained from most op-amps.

The Cs on the first transistor are input interference filters.

Bias voltages might seem a trifle difficult to discover but the clue here is R2 whose voltage supplies bias via R10/12 to Q1.

There is only one AC negative feedback loop which is via R8. All other negative feedback is at DC due to bypass capacitors.

Input slew rate is determined by R11, 8 which form the factor "m" in S = 0.3mFt. This is an op-amp formula but works for this circuit too. The expected slew rate for its bandwidth would be quite low otherwise.

Unlike op-amps this stage is all class-A.
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