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Old 9th October 2015, 06:22 AM   #1
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Default Thanh Amplifier

I am trying to model the amp attached.
When I use the ideal opamp without power it works fine.
When I use the ideal opamp with power it gives a unit step looking result down to around -30V
When I use an opamp model it gives the same unit step looking result.

I have tried with and without C1 and checked for proper voltage source voltage.

Any ideas?
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File Type: png schematic.png (32.4 KB, 460 views)
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Old 9th October 2015, 08:03 AM   #2
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Now ideal opamp with power works. Drop in an opamp model and it doesn't
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Old 9th October 2015, 08:06 AM   #3
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WOrking with a (fourth or fifth) opamp model I hadn't tried before. My bad. Can anyone suggest which transistors to use? I am not familiar with enough of them to know.

Last edited by vladimirb0b; 9th October 2015 at 08:27 AM.
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Old 9th October 2015, 10:41 PM   #4
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For starters, I'd try something like BC556B/BC546B (or 2N5551/2N5401), BD139/140 and TIP41C/TIP42C (or D44H11/D45H11, or even TIP2955/TIP3055).

It also helps if there aren't any transistors with floating emitters, like the middle pnp on the first schematic. Why doesn't it have a designator anyway?

Finally a small capacitor in the double-digit pF range between opamp pins 6 and 2 may prove useful.

Oh, wait a minute... your output stage is overall inverting (look at the CFP, it's not a buffer!), so you must reverse the input and feedback connections on the opamp (leave the lil' cap where it is though). Otherwise you'll get positive feedback and things instantly latch up as you've seen. I guess the generic opamp model (sans power) had the input pins swapped? Bit of a trap there.

I really wonder how that circuit works without any kind of extra compensation. Don't be surprised if you find it has all the stability of a house of cards.
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Old 10th October 2015, 01:42 AM   #5
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It is simming well with the default opamp configuration. SOme opamps work, some don't.

I tried with and without the "floating" emitter and it made little difference. So I left it connected.

The stage doesn't seem inverting. It gives proper phase.

Which transistor would go where? It is a pain to figure out wattage in Tina and I haven't converted it to LTSpice.

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Old 10th October 2015, 01:49 AM   #6
kevinkr is offline kevinkr  United States
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The power stage appears to be an inverting configuration to me too, try swapping the inputs to the op-amp.
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Old 10th October 2015, 02:01 AM   #7
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Here is the phase response. Doesn't seem inverted to me.
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Old 10th October 2015, 07:16 AM   #8
tomchr is offline tomchr  Canada
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Can the opamp even supply the required current? The input impedance to the two emitters should be pretty low and there's only 10 Ω in series.

Referring to the first schematic (LTspice): If you pull down on R21, more current flows in Q2, correct? This would pull down the base of the top PNP (Q3?) and cause the two NPNs to be pulled high and the output high. That's inverting, right? If so, as Kevin said, you need to swap the connections at the input to the opamp.

I suggest simulating the output stage first. Once you have that working, bring the two halves together.

You may find this thread interesting: composite amplifier design: advice and thoughts before I start please.
I think the OP ended up settling on attaching a CFP output stage to an opamp.

Modulus-86: Composite amp achieving 0.000067 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
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Last edited by tomchr; 10th October 2015 at 07:19 AM.
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Old 10th October 2015, 05:52 PM   #9
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Put a ammeter at the output of the opamp and is basically 0
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Old 10th October 2015, 06:45 PM   #10
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Maybe I need to switch to non-ideal transistors. Still hoping someone can tell me which go where.
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