Help in understanding circuit

Thread Starter

dineshkakubal

Joined Aug 18, 2020
5
I am using opamp OPA1664 as a voltage follower for one application. I am using a filter circuit for the output as shown below.
1597762308785.png
I have copied the circuit from some reference design. I do not have idea what the circuit at the output is. Can anyone help me understand the circuit and how to select the values for diode, transistor and resistor?
 

dl324

Joined Mar 30, 2015
11,926
Welcome to AAC!

It makes no sense to me. Are you certain you drew it correctly? It would make more sense for the opamp to be driving a class AB amplifier.

As drawn, it's going to damage Q2. The BE junction will breakdown and the current gain will be decreased permanently.
 

OBW0549

Joined Mar 2, 2015
3,566
I am using opamp OPA1664 as a voltage follower for one application.
Yes, the voltage follower is evident on the left hand side of your diagram. So far, so good.

I am using a filter circuit for the output as shown below.
Whatever that circuit is, it is certainly NOT any kind of filter.

I have copied the circuit from some reference design.
Either you made numerous, HUGE mistakes copying it, or the original "reference design" was made by an idiot.

I do not have idea what the circuit at the output is.
I don't either; it's such a mess that it's impossible to tell with any certainty what it was intended to do. If I had to take a wild guess, it looks like it might have been intended as some sort of limiter, or clamp; but as it is, it could not possibly work that way-- or any other way.

Can anyone help me understand the circuit and how to select the values for diode, transistor and resistor?
The only thing you need to understand about this circuit is that it is garbage, and cannot be made to work no matter what diodes, transistors or resistors you choose. It is a hopeless mess.

My recommendation: be more careful where you copy circuit designs from.
 

Thread Starter

dineshkakubal

Joined Aug 18, 2020
5
Welcome to AAC!

It makes no sense to me. Are you certain you drew it correctly? It would make more sense for the opamp to be driving a class AB amplifier.

As drawn, it's going to damage Q2. The BE junction will breakdown and the current gain will be decreased permanently.
Hi Dennis,
Thanks for the insight.
This circuit is given by customer and asked to copy the same.

Here is their explanation of the circuit:
Circuit Theory of Operation:
1. The series resistor between the op amp output and the connector in each VOUT_xx circuit will introduce a voltage output between the voltage applied to the connector and the op amp output
2. If / when the voltage at the output of any op amp output exceeds
+15VDC plus, nominally, two forward diode drops, the protective
PNP transistor will turn on
3. In the on state, the protective PNP transistor provides a path to ground, limiting the maximum voltage seen at the output of any op amp
4. If / when the voltage at the output of any op amp output is less than -
15VDC plus, nominally, two forward diode drops, the protective
NPN transistor will turn on
5. In the on state, the protective NPN transistor provides a path to ground, limiting the minimum voltage seen at the output of any op amp
6. It is possible that the bas reference voltage on the PNP and NPN
transistors may need to be +/-- 13.5V respectively
7. It is also possible that protection against -24VDC may not be required based on the ability to reverse the +24VDC / GND connector input
8. Specific component PNs require detailed circuit design including maximum +/-24V current that may be connected


Thanks,
Dinesh KS
 

crutschow

Joined Mar 14, 2008
25,686
2. If / when the voltage at the output of any op amp output exceeds
+15VDC plus, nominally, two forward diode drops, the protective
PNP transistor will turn on
How can you get an output above 15V if the op amp power supply is 15V?

In any event, it's poor practice to brute-force clamp the output of an op amp.
Better to use a negative feedback clamp.
 
Last edited:

RPLaJeunesse

Joined Jul 29, 2018
124

ronsimpson

Joined Oct 7, 2019
1,024
I think the circuit does not work in part because Q1 and Q2 are configured very differently. Q2 has the Emitter sitting on ground. Q1 has the Emitter, more or less connected to the op-amp. Now if I know which was right .....
 

panic mode

Joined Oct 10, 2011
1,865
Q2 has C-E is connected reversed. maybe entire circuit is drawn incorrectly (or designed incorrectly).

i guess someone tried to protect the Vout from getting to some unsafe values. OpAmp cannot bright the Vout to such values since it is powered from +/-15V as well but maybe there is more to it. (driving inductive load or something else?)

either way transistors and the 0 Ohm resistors don't do anything, should have just used diodes if clamping was desired. they alone would be better clamp.
 

Rod777

Joined Aug 24, 2019
3
Unfortunately I have been out of the trade for a number of years now, but the input would have to be greater than +/- 15V to get the transistors to do anything. If either transistor were to turn on then both it and its diode might let out the magic smoke.
What is the input.?
 

MrChips

Joined Oct 2, 2009
22,113
It looks like someone attempted to reverse engineer a push-pull power output stage and drew the circuit incorrectly including the base-collector-emitter pinout of the power transistors.

Change the orientation of the transistors to base-collector-emitter, put a loudspeaker between VOUT and AGND and it would start to make some sense.
 

Audioguru again

Joined Oct 21, 2019
2,090
The #1 in the specifications says that the voltage coming in from the output connector will be limited to the supply voltage plus two diode drops.
But the datasheet for the opamp does not say if the opamp will be damaged by having its output driven externally above the supply voltage.

The audio amplifier shown by MrChips will have a very low output level and have severe crossover distortion.
 

MrChips

Joined Oct 2, 2009
22,113
The #1 in the specifications says that the voltage coming in from the output connector will be limited to the supply voltage plus two diode drops.
But the datasheet for the opamp does not say if the opamp will be damaged by having its output driven externally above the supply voltage.

The audio amplifier shown by MrChips will have a very low output level and have severe crossover distortion.
Geez. I'm just saying someone drew the circuit wrong. I don't know what other components are missing from the drawing.
 
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