Help With This OpAmp

[Samuel Groner]

Hi

Having the latest non-working incarnation of one of my discrete opamps on the bench: [removed]

A few (probably irrelevant) corrections on the schemo:
* R1, R2, R9 = 91 ohm
* C1a open
* C1b = 100 pF in serie with 120 ohm
* C2 open
* Vsupply = +/- 15 V

The diff. input stage works OK, except that the right leg conducts a bit more than the left (voltage drop across R3 = 460 mV and across R4 = 530 mV). Across R7, I measure around 600 mV, the second CCS is saturated (@ 1.3 mA or so), D5-D7 do not conduct and the output stage is (obviously) dried out as well.

I replaced Q9, no change and the unsoldered transistors seems to be OK. Shorting Q9 emitter to collector makes things going and the second CCS as well as the the output stage run at a good quiescent current.

Any hints where to look next?

Thanks!

Samuel

 

[bcarso]

What happens if you open-circuit the feedback divider components? That is, set it up for unity d.c. and a.c. gain.

Note that when you do have a large negative d.c. output voltage the three 1000uF lytics are reverse-biased and will leak like crazy and also to some extent take a reverse charge. If that kind of accident occurs, when recovering it will be a long time correcting at low gain settings (10k * 3000uF = 30 seconds).

You could also disconnect the input stage from the base of Q8 and run some current to common or -V through a resistor to see if everything moves from that point on. With the base of Q8 open you should have the output voltage far negative, and with a tiny bit of current towards ground it should swing far positive.

 

[tk@halmi]

Across R7, I measure around 600 mV

I wonder if that is enough to get Q9 really open.

Try to simplify. You don't need everything to get all the transistors to bias up. Remove D4, C1a and C1b from the VAS for the time being. Replace D3 with a 100 to 200 ohm resistor and test. Bypass C3s until the circuit gets going. When it works gradually add the components back.

 

[Samuel Groner]

Thanks for the suggestions! Still nothing, unfortunately...

Replaced Q10 (done that before posting here, in fact), shorted C3, removed D4, replaced D3 with 130 ohm - no change.

Finally removed Q8 (I'll update my TO-92 footprint, that's for sure...) and still no change! My DVM measures 0 mV across R7, so no base current flowing for Q9. :? If I measure from the + supply to the base leg of Q9, I get 2k, so all connections are OK.

Oh, and trimming R14 doesn't influence things at all. Before removing Q8, the output sits at 0 V after some dozend seconds, but the supply rails are asymmetrical (I have a single 30 V floating supply only) regarding ground, about -5/+25 V.

Any ideas?

Never mind, that's how we learn!

Samuel

 

[bcarso]

[quote author="Samuel Groner"]

Oh, and trimming R14 doesn't influence things at all. Before removing Q8, the output sits at 0 V after some dozend seconds, but the supply rails are asymmetrical (I have a single 30 V floating supply only) regarding ground, about -5/+25 V.

Samuel[/quote]

How are you establishing what common is then? Is there some stiff voltage divider not shown?

If there is nothing to assert a half-supply potential, that could be your problem.

 

[Samuel Groner]

That's what I thought as well and so I used a voltage divider and grounded to the chassis of my drilling machine. This didn't change anything, so I removed this components again.

Today I powered it with a proper +/- 18 V dual from another not finished project - again no change. Q8 was still removed, though.

Samuel

 

[Samuel Groner]

Latest report, Q8 back in and D8/D9 removed:
Output at -16 V, then paralleled R8 with a 9.7k resistor; this made the second CCS run @ 5 mA and the output @ 15 mA, already much better. But not there yet, output still at -16 V...

Samuel

 

[bcarso]

[quote author="Samuel Groner"]Latest report, Q8 back in and D8/D9 removed:
Output at -16 V, then paralleled R8 with a 9.7k resistor; this made the second CCS run @ 5 mA and the output @ 15 mA, already much better. But not there yet, output still at -16 V...

Samuel[/quote]

Did you do the experiment to see if the stages after the input stage work properly?

I would test with the proper split supply in any case---the lack of a centering circuit for common is a definite problem, regardless of your other problems.