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# Understanding Purpose of Feedback in Circuit

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#### Zara Zara

##### Junior Member level 1
Hello Guys,

In the circuit below , I get this part of a big circuit.

This circuit has an input of trianguler with a random offset , it's compared with the zero level and a pwm output is fed to push pull transistors,

until now , no problem.

But why the feedback exists in this circuit?

Can anyone help !

thank you

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Hi,

this is a dubious circuit.
* Why the +/- 200V? because the output swing is less than the OPAMP output swing. Maybe +/- 10V.
* if the signal source is low impedance then the bottom circuit is useless. If it is high impedance, you need to know the impedance.

Klaus

Hey Klaus !

why would the bottom circuit be useless if the input source is low impedance ?

Is this feedback a kind of stabilization ?

Hi,

if the source is low impedance, then the voltage on U1/pin 2 only depends on input signal and not on R9 current.

and the other way with it´s series impedance of more than 1.5MOhms it won´t significally change the output signal with an impedance of abot 1500 Ohms.

So neither the right side of R9 can change the OUT put signal, nor the left side of R9 can change the input signal. --> no function at all.

******
But if input impedance is high (maybe100k) then the current through R9 will change the signal into U1/pin 2.
--> You need the input impedance (source impedance) to calculate the circuit.

Klaus

Each opamp needs negative feedback for operation within the linear region.
However, a low-impedance signal source at the inv. input would short the feeback signal.
The source impedance does not need to be very high - perhaps some kohms.

Hi,

Each opamp needs negative feedback for operation within the linear region.
True for linear operation, but the OP talks about PWM output. Comparator style.

... triangular input with random offset....
The feedback is only for very low frequencies. It may keep the DC offset in a linear region (for a very limited DC input range) but there is about no feedback for AC like the triangle.
Therefore i think even a some mV triangle will bring the OPAMP output into saturation.

The schematic is a riddle....for me the circuit makes no sense...

Klaus

Hi

Klauss you said that
but there is about no feedback for AC like the triangle

so if the triangular signal is an sinusoidal signal the issue is different here .

thank you

The schematic is a riddle....for me the circuit makes no sense...
You can add a series resistor for the "Triangular with offset" signal and get a pulse width modulator with DC feedback.

Besides the concerns already mentioned, the BU208 is not suitable, it's complimentary PNP version probably doesn't exist. The last time I saw one was maybe 35 years ago, they were used in early color TV EHT/scan generators. Vbe = typically 1.3V and Hfe of 2.5 suits their original purpose in pulsed applications but not a linear circuit.

Brian.

I think it's more or less pointless to discuss the details of an obviously flawed circuit design. The circuit makes sense with a a low voltage (e.g. +/-15 V) supply, otherwise it seems to represent the idea to switch somehow +/- 200V, controlled by a low voltage comparator. As said, it doesn't work this way. But there are possible circuits and complementary BJT are available with at least 350 V rating.

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