strange op-amp circuit

kubeek · 14th October 2006, 09:27 PM

I just listed through a complex 480 pages long book about op-amps, but I didn´t find this circuit.It is shown in few Fender amp schematics, it´s not connected in signal chain, but on the path to CA3080 in compressor circuit.
Can anyone tell me what is it good for, and why is it connected the way it is?
I have also plotted the in(green)/out(blue) curve.

kubeek · 14th October 2006, 09:28 PM

and the graph:

FastEddy · 14th October 2006, 09:48 PM

Unity gain, inverting Op-Amp circuit with + & - terminals reversed top to bottom from "common practice".

(Everything you wanted to know about Op-Amps but was 'fraid to ask = http://www.tinaja.com/ ... When he was a lad of 20, he began writing Op-Amp Cookbooks circa 1970 ... eventually becoming an engineer for National ... now quite independant and full of crazy ideas about "perfect" sign waves, etc. ...

)

kubeek · 14th October 2006, 09:51 PM

I don´t understand, you want to say it has unity gain? No it doesn´t.

FastEddy · 14th October 2006, 09:57 PM

OK, so what is the gain? (I assumed that with 560 ohms on both the top and bottom, inverting and non-inverting feedback loops, the gain of The Amp would be unity ... granted there is resistance of 5.6K on both inputs, but The Amp is set to unity = 1 = gain. ... Overall circuit might produce something like a negative "gain" of -1.8 to -2.x or more (= -less).

kubeek · 14th October 2006, 10:01 PM

You see the graph, the ouput changes 24volts for cca 1volt input change, that´s a "gain" of 24 (+/- somehting).
I don´t understand this circuit, but I think the ouput product highly depends on input offset voltage.

kubeek · 14th October 2006, 10:11 PM

The output doesn´t depend on offset voltage, nor offset current. Changing R2 changes the slope, other resistors just move the ouput "left/right".

FastEddy · 14th October 2006, 10:13 PM

Yes, yes ... yes, now I see what you mean.

This thing is set up like a Schmidt Trigger .. as soon as the input voltage crosses ~= +0.6 volts (input transistor threshold), it flips over to the positive rail voltage ... and vise versa. A voltage follower with high gain = Schmidt Trigger = a common "flip flop" circuit since the invention of TTL logic ala TRW (Dr. Ramo = the R in TRW).

(I am totally wrong about the circuit gain and the Op-Amp gain ... without the input resistors this would probably oscilate or latch to one rail or the other = gain >>= 10X.)

pinkmouse · 14th October 2006, 10:15 PM

Where in the circuit is it? What is it connected to? It looks to me that it is amplifying a lot and driving the output to the rail limits with a small input. Almost like some sort of square wave generator.

FastEddy · 14th October 2006, 10:16 PM

" ... It looks to me that it is amplifying a lot and driving the output to the rail limits with a small input. Almost like some sort of square wave generator. ..."

Exactly.

14th October 2006, 10:15 PM	#9
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Where in the circuit is it? What is it connected to? It looks to me that it is amplifying a lot and driving the output to the rail limits with a small input. Almost like some sort of square wave generator. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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14th October 2006, 09:48 PM	#3
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	Unity gain, inverting Op-Amp circuit with + & - terminals reversed top to bottom from "common practice". (Everything you wanted to know about Op-Amps but was 'fraid to ask = http://www.tinaja.com/ ... When he was a lad of 20, he began writing Op-Amp Cookbooks circa 1970 ... eventually becoming an engineer for National ... now quite independant and full of crazy ideas about "perfect" sign waves, etc. ... )

14th October 2006, 09:51 PM	#4
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	I don´t understand, you want to say it has unity gain? No it doesn´t.

14th October 2006, 09:57 PM	#5
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	OK, so what is the gain? (I assumed that with 560 ohms on both the top and bottom, inverting and non-inverting feedback loops, the gain of The Amp would be unity ... granted there is resistance of 5.6K on both inputs, but The Amp is set to unity = 1 = gain. ... Overall circuit might produce something like a negative "gain" of -1.8 to -2.x or more (= -less).

14th October 2006, 10:01 PM	#6
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	You see the graph, the ouput changes 24volts for cca 1volt input change, that´s a "gain" of 24 (+/- somehting). I don´t understand this circuit, but I think the ouput product highly depends on input offset voltage.

14th October 2006, 10:11 PM	#7
kubeek diyAudio Member Join Date: Dec 2005 Location: Prague, Czech Rep	The output doesn´t depend on offset voltage, nor offset current. Changing R2 changes the slope, other resistors just move the ouput "left/right".

14th October 2006, 10:13 PM	#8
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	Yes, yes ... yes, now I see what you mean. This thing is set up like a Schmidt Trigger .. as soon as the input voltage crosses ~= +0.6 volts (input transistor threshold), it flips over to the positive rail voltage ... and vise versa. A voltage follower with high gain = Schmidt Trigger = a common "flip flop" circuit since the invention of TTL logic ala TRW (Dr. Ramo = the R in TRW). (I am totally wrong about the circuit gain and the Op-Amp gain ... without the input resistors this would probably oscilate or latch to one rail or the other = gain >>= 10X.)

14th October 2006, 10:16 PM	#10
FastEddy diyAudio Member Join Date: Jun 2006 Location: North Californie	" ... It looks to me that it is amplifying a lot and driving the output to the rail limits with a small input. Almost like some sort of square wave generator. ..." Exactly.

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