[sdiy] transistor op-amp problems

Needham, Alan Alan.Needham at centrica.com
Fri May 25 18:18:13 CEST 2012


If I understand you correctly, you are measuring from the negative
supply rail (emitters of Q5 and Q6)?
If this is the case, I think that is your problem.
Try using the centre tap of the two 6v batteries instead. As long as
your input is as big as 1v WRT the supply centre tapping your output
will hit a power rail so reduce it to 0.2v - the output should then be
about 2.2v WRT the supply centre tap.

That 'earth' symbol is misleading.

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Elain Klopke
Sent: 25 May 2012 16:01
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] transistor op-amp problems

Hey all,

I built the simple op-amp in the non-inverting amplifier setup shown
at the bottom of this page:
http://www.allaboutcircuits.com/vol_6/chpt_5/17.html

I used 2N3906s for the PNP transistors and 2N3904s for the NPNs and
put a 10Kohm reistor in the feedback loop and a 1Kohm resistor to
ground from the inverting input. In theory that should have produced a
gain of 11. However, when I adjust the pot so that one volt is going
into the non-inverting input of the circuit, I get 1.04 volts at the
output. I measured between the wiper of the pot and the ground
connection and between the output and ground. I can't quite figure it
out.

-Ian Smith
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