[sdiy] transistor op-amp problems

Phil Macphail phil.macphail at liivatera.com
Sun May 27 22:58:06 CEST 2012

Hi Elain,
Sorry for the delay in replying, I have been away from home for a few days
but here goes
Breaking the problem into steps for clarity -
1. The resistor Rprg is 100k with a little over 10Volts across it. This
gives a little over 100uA into Q5, and is the bias circuit I was referring
to. Reduce this resistor value to run the design at a higher current.
2. This 100uA is mirrored by Q6, so each side of the differential pair
will carry 50uA when balanced, so with no load the current mirrored by
Q1,Q2 will be equal to the collector current of Q4 and the output voltage
doesn't change. 
3. Any load on the output (such as the resistors in red) will unbalance
the differential pair - the limit is when Q4 isn't conducting and Q3
carries the full 100uA. In this case the 100uA is mirrored by Q1,Q2, and
there is no current flowing through Q4.
4. The maximum current into the feedback resistors and load is therefore
5. With the resistors you chose (10k + 1k) with 100uA flowing through them
gives a voltage of 1.1Volts - assuming all transistors are perfectly
matched - which is pretty much the voltage you measure.

So if you want a larger output voltage range you will need to follow the
suggestions in my first mail. To make a useful amplifier from this circuit
you need to add a buffer to the output, so this circuit is really of
educational value only (with the limitations others mentioned too).

On 26/05/2012 05:47, "Elain Klopke" <functionofform at gmail.com> wrote:

>On Fri, May 25, 2012 at 2:46 PM, Phil Macphail
><phil.macphail at liivatera.com> wrote:
>> If you have used the same bias circuit, then the amplifier can only
>>source/sink around 100uA. Into 10k that is about 1 volt,
>> Either increase the bias current or increase the load impedance ( or
>>both), add a buffer or other gain stage and it should start to work,
>> Good luck,
>> Phil
>You're probably going to have to show me a diagram of what you're
>talking about Phil.... Just so I can make sure I get it.
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