Operational Amplifier design help

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elrhazouani

Joined Feb 8, 2011
15
This is what I came up with so far, a gain of about 28. In class, the only purpose of using the darlington pair was to give a high gain. and the hand calculations showed that there was a differential gain of around 5000 V/V for a problem that has the same setup as the attached circuit. can you explain why I get such a fair gain in here, and how to make it better?
 

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thatoneguy

Joined Feb 19, 2009
6,359
What was the circuit in class?

I'm not the wizard of input stages, and usually use FETs for switching.

Hopefully others will be along as well, though, but I'll try to bump along, did you read the link I posted above?
 

Adjuster

Joined Dec 26, 2010
2,148
Those FETs are operating at only some tens of microamps. At that level their gm will be low. You may get better results with with more current, although the drain loads will need to be reduced to maintain acceptable voltages.

Generally junction FETs will not give a large enough gm relative to the drain current to permit extreme voltage gains in a resistively loaded stage. The use of a Darlington pair following stage in principle allows a high impedance load to be used, but the DC drain current limits the practical load resistance.

If a large gain is actually required, consider the use of a cross-coupled current mirror load. This might be implemented using P-channel FETs or PNP BJTs, but note that the bias voltage stability of such things tends to be poor. An overall negative feedback loop may be required to establish a stable working point.
 

Audioguru

Joined Dec 20, 2007
11,249
The darlington-connected transistors have NO voltage gain. Their output voltage swing is a little less than the input voltage swing if they are not loaded much. They amplify current so that the differential amplifier has a very high load resistance so it can have high voltage gain.

You are making an opamp but your circuit barely looks like the schematic of thousands of opamps or audio amplifiers. Why don't you look at one?

Here is a simple audio amplifier from Red Circuits. It can perform like a simple opamp and do what you want.
 

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thatoneguy

Joined Feb 19, 2009
6,359
Here is a simple audio amplifier from Red Circuits. It can perform like a simple opamp and do what you want.
This is close to being what would work, except the assignment doesn't allow capacitors. The input removes DC bias, so it can be removed, while C2 and C3 are for frequency response and can be safely removed for this specific assignment.

What to do about C4 bootstrapping the lower output driver, though? A voltage divider bias replacement of some sort?
 

Audioguru

Joined Dec 20, 2007
11,249
This is close to being what would work, except the assignment doesn't allow capacitors. The input removes DC bias, so it can be removed, while C2 and C3 are for frequency response and can be safely removed for this specific assignment.
C1 is the input coupling capacitor which can be removed.
R1 biases the input at 0V and it can also be removed.
R2 and C2 cut radio frequency interference and can be removed.
C3 allows a DC gain of 1 for good bias stability but a high AC gain and can be removed.

What to do about C4 bootstrapping the lower output driver, though?
C4 is a simple integrator that cuts high frequencies (the frequency compensation capacitor in almost every opamp) so that plenty of negative feedback can be applied without oscillation happening. It can be removed.

If this opamp has a gain of 2000 and is DC-coupled (because capacitors are not allowed) then it will saturate if the input DC is +1V and -1V.
 

thatoneguy

Joined Feb 19, 2009
6,359
Thank you for the detailed explanation! Audio amps and op amps aren't my forte, and you explained things better than I.

I thought C4 was providing bias via bootstrap for some reason, but now I realize there's no resistors/other components with it. :rolleyes:

I'm assuming the inputs would be on the two bases of the LTP?

--ETA: To the OP:

Your output is riding on a 2.065V bias, which can be removed by using the push-pull output (actually, the entire circuit) posted by audioguru above, omitting the caps as he stated.

The circuit posted by audioguru is a current source feeding a long tailed pair, the output of which goes into a push-pull complimentary pair, it is bare-bones simple compared to the composition of any op-amp IC, and probably about the simplest solution to fit your requirements.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=37024&d=1322495975 (your output)
 
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Adjuster

Joined Dec 26, 2010
2,148
The OP needs to amplify the AC content but not the DC, without capacitors. Perhaps inductances or transformers are permitted?

Failing that, possibly frequency dependencies in semiconductors could be used. Would using a reverse biased diode or transistor junction capacitance as a coupling constitute "cheating"?

Then again, DC feedback might perhaps be applied around an amplifier using a feedback path designed to have very low bandwidth, so that at moderate frequencies a much larger gin is obtained. It might be fun trying to maker such a system stable.
 
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