pseudo-opamp behaviour - schematic analysis

Thread Starter

Mario777

Joined Nov 17, 2023
12
Hi All,
I have a schematic (attached) which from the outside perspective behaves like an opam but what I am trying to do now is actually understand its behaviour as struggling with it. There are two main current sources on BJTs Q3 and Q7 stage. Q3 stage delivers 1mA current to an amplifier Q1/Q2. This is what I do understand what I do not however is the cascaded behaviour of Q(4,5,6,8,9). Please help
 

Attachments

Thread Starter

Mario777

Joined Nov 17, 2023
12
What precisely is an opam? Sounds like what my grandson called me when he was learning to talk.
It is an operational amplifier what I said was to indicate the feedback associated with this black box which I have simulated. But what I do not understand is the transistor network associated with the output.
 

MisterBill2

Joined Jan 23, 2018
17,800
I am wondering where this circuit came from because it does not look quite right.
What is it claimed to do?? How is it claimed to be better than a different circuit.
ALSO, as it seems to lack a major op-amp feature of both inverting and non -inverting inputs, how can it be an op-amp?
Did it appear on youtube?
 

Papabravo

Joined Feb 24, 2006
20,996
It is an operational amplifier what I said was to indicate the feedback associated with this black box which I have simulated. But what I do not understand is the transistor network associated with the output.
OK - sorry filling in the letter "p" was beyond my meager capabilities.
I'm not sure how your circuit was designed, and it looks similar to an audio amplifier, but it does not seem like it will be able to function properly. Do you have some information suggesting it does work?

A typical class AB output stage would have a NPN/PNP pair with the emitters tied together, like this:
1701139427921.png
 
Last edited:

Thread Starter

Mario777

Joined Nov 17, 2023
12
I am wondering where this circuit came from because it does not look quite right.
What is it claimed to do?? How is it claimed to be better than a different circuit.
ALSO, as it seems to lack a major op-amp feature of both inverting and non -inverting inputs, how can it be an op-amp?
Did it appear on youtube?
No it is not a youtube circuit I got it by accident but since my major used to be digital when I worked in the field I have forgotten the basics just trying to verify the functionality of the circuit, I suspect there is something wrong with it thats way I decided to ask you guys here for your opinion.
 

Thread Starter

Mario777

Joined Nov 17, 2023
12
OK - sorry filling in the letter "p" was beyond my meager capabilities.
I'm not sure how your circuit was designed, and it looks similar to an audio amplifier, but it does not seem like it will be able to function properly. Do you have some information suggesting it does work?

A typical class AB output stage would have a NPN/PNP pair with the emitters tied together, like this:
View attachment 308577
I am not familiar with amplifiers as such I can identify main stages from what I remember from uni. Just trying to get the idea behind this one here. As far as I got with it was to put some sin wave through it and it seems to be amplifying the signal but I can not determine the functionality of it. I will do some research on audio amplifiers just though you guys here could identify it and help.
 

Papabravo

Joined Feb 24, 2006
20,996
I am not familiar with amplifiers as such I can identify main stages from what I remember from uni. Just trying to get the idea behind this one here. As far as I got with it was to put some sin wave through it and it seems to be amplifying the signal but I can not determine the functionality of it. I will do some research on audio amplifiers just though you guys here could identify it and help.
Here is a basic article on amplifier classes.
Amplifier Classes and the Classification of Amplifiers (electronics-tutorials.ws)
These guys have other useful reference pages.
 

AnalogKid

Joined Aug 1, 2013
10,944
The overall circuit is an inverting amplifier with a gain of 10.

Q8 and Q9 form a complimentary-symmetry output stage, very common in audio power amplifiers. As pointed out, Q8 is upside down. This is a class AB stage.

Q6 and PT2 form what is sometimes called an active zener, zener multiplier, variable zener, Vbe multiplier, or a rubber diode. The collector-emitter voltage is constant regardless of the collector current. The voltage is set by PT1 acting as both legs of a voltage divider. This is adjusted to minimize the crossover distortion in the output stage.

Q4 is the main voltage amplifier for the circuit. In most IC opamps, it has a small capacitor from collector to base, called a Miller capacitor, to improve stability at low circuit gains when the circuit's gain is at its max.

Q5 is interesting. It is a common-base amplifier as the Q4 collector load. With Q4 it forms a cascode amplifier. This has higher gain and greater bandwidth than a single transistor voltage amp, but it does reduce the max output voltage amplitude. Per the Wiki, it eliminates the need for the Miller capacitor.

https://en.wikipedia.org/wiki/Cascode

ak
 
Last edited:

Ian0

Joined Aug 7, 2020
9,499
The overall circuit is an inverting amplifier with a gain of 10.

Q8 and Q9 form a complimentary-symmetry output stage, very common in audio power amplifiers. As pointed out, Q8 is upside down. This is a class AB stage.

Q6 and PT2 form what is sometimes called an active zener, zener multiplier, variable zener, Vbe multiplier, or a rubber diode. The collector-emitter voltage is constant regardless of the collector current. The voltage is set by PT1 acting as both legs of a voltage divider. This is adjusted to minimize the crossover distortion in the output stage.

Q4 is the main voltage amplifier for the circuit. In most IC opamps, it has a small capacitor from collector to base, called a Miller capacitor, to improve stability at low circuit gains when the circuit's gain is at its max.

Q5 is interesting. It is a common-base amplifier as the Q4 collector load. With Q4 it forms a cascode amplifier. This has higher gain and greater bandwidth than a single transistor voltage amp, but it does reduce the max output voltage amplitude. Per the Wiki, it eliminates the need for the Miller capacitor.

https://en.wikipedia.org/wiki/Cascode

ak
Shouldn’t the dominant pole compensation capacitor go from Q5 collector to Q4 base?
Q5 emitter (and therefore Q4 collector) will be at a fixed voltage, because Q5 base is at a fixed voltage.
As the purpose of cascode is to eliminate the Miller effect, adding a “Miller” capacitor makes the use of a cascode look rather daft.
However, it allows the use of a small transistor with low Vceo and low power dissipation to be used as Q5, and a higher voltage higher power transistor to be used as Q5, because Q5 has to dissipate some power.
 

Ian0

Joined Aug 7, 2020
9,499
My description of the Miller capacitor was for the more common, non-cascode voltage amplifier stage. Speaking of the cascode amplifier in this application,

ak
Nonsense. It eliminates the miller capacitance of the voltage amplifier stage.
That doesn’t mean that it will work without a dominant pole capacitor.
 

MisterBill2

Joined Jan 23, 2018
17,800
OK, as noted by comments from others, it does not seem to be quite right, that is why I asked if it came from yoo tube, which I regard as a source of incorrect stuff, although evidently sometimes it has correct information.
But certainly it is not an Operational Amplifier.
 

Ian0

Joined Aug 7, 2020
9,499
But certainly it is not an Operational Amplifier.
it most definitely is an operational amplifier (with one transistor upside down). It has inverting and non-inverting inputs on the bases of Q1 and Q2, high gain and a push-pull output. It looks very much like the discrete op-amp circuits from Linsley-Hood’s book.
 

Ya’akov

Joined Jan 27, 2019
8,973
I have corrected the spelling of pseudo from “sudo” in the title of this thread.

It’s not terribly important, but sudo is a Linux program to manage user access to root privileges and I kept trying to understand what that had to do with an OPA.
 

MisterBill2

Joined Jan 23, 2018
17,800
it most definitely is an operational amplifier (with one transistor upside down). It has inverting and non-inverting inputs on the bases of Q1 and Q2, high gain and a push-pull output. It looks very much like the discrete op-amp circuits from Linsley-Hood’s book.
The base of Q2 is not shown as an input connection, at least not in post #1.
At first glance it does look like an op-amp, but I did not assume an input where none was shown.
 

AnalogKid

Joined Aug 1, 2013
10,944
The base of Q2 is not shown as an input connection, at least not in post #1.
As flagged in the title, note that the #1 schematic is not presented as the internal schematic of an opamp component; it is that of an opamp *circuit*. In this case, an inverting amplifier with a gain of 10 set by R11 and R12. Q2 is the non-inverting input, tied to GND as it should be for a dual-supply circuit in this application.

ak

Semantic sidecar: Is it an inverting amplifier with again of 10, or an inverting amplifier with a gain of -10? Let's ignore the cop-out "amplifier with a gain of -10."
 
Last edited:

AnalogKid

Joined Aug 1, 2013
10,944
Q3 and Q7 are lacking a base-bias source, so obviously the circuit won't work without that.
R2 is connected to GND, and the diode string and the two current source emitters are connected to -30 V. This establishes a bias voltage of around -28 V, and a bias current through the diode string of about 50 uA. Seems a bit low to me, but, whatever. C2 decouples the DC bias for a nice low impedance at those pesky AC frequencies.

ak
 
Last edited:
Top