error amplifier vs comparator

[--BawA--]

What is the difference between error amplifier and a comparator?

 

[Audioguru]

An error amplifier is a linear amplifier. A comparator output digitally switches high or low with nothing in between.

 

[--BawA--]

But the configuration of both are same , i mean error amplifier has open loop gain and comparator has also.(ignore compensation).
both has Vref,
then how error amp works in linear region ?

 

[Dominik Przyborowski]

then how error amp works in linear region ?

amplifiers working with negative feedback

 

[Audioguru]

An error amplifier NEVER has an open loop gain. The open loop gain of an opamp is 200,000 to a few million. The error amplifier always has a much lower closed loop gain which might be 1.

An example is a linear voltage regulator. If the input increases 1V and a load tries to reduce the output by 1V then the error is 2V. An opamp with an open loop gain of 200,000 is used as a linear error amplifier and it reduces the error at the output to 10mV.
The same for a linear audio amplifier. The high current in the speaker tries to reduce the output swing by 2V. The error amplifier again reduces it to 10mV.

A comparator is never used closed loop (with negative feedback) because it has no frequency compensation capacitor so negative feedback will cause it to oscillate at a high frequency.

 

[--BawA--]

you mean to say that , in error amplifier, compensation network act as a negative feedback, so how to calculate the gain of the error amp?

 

[Audioguru]

The compensation capacitor in an opamp has nothing to do with its negative feedback. The compensation cuts high frequencies so that it reduces gain to less than 1 at the high frequency where the opamp has enough phase shift to cause it to oscillate when negative feedback is added. Negative feedback is usually made with a wire (closed loop gain of 1) or with two resistors (any closed loop gain you want).
A comparator does not have a compensation capacitor in it so it oscillates at a high frequency when negative feedback is added.

You cannot calculate the open loop gain of an error amplifier. Look on its datasheet.

 

[--BawA--]

Negative feedback is usually made with a wire (closed loop gain of 1) or with two resistors (any closed loop gain you want).

In the above schematics , please see the highlighted portion, TL431 is acting as an error amplifier , but there is no resistive feedback network , however compensation network of series RC is used. Please through some light on it.
How the gain has been set in the above error amplifier(TL431) ?

 

[Audioguru]

The TL431 is an adjustable shunt voltage regulator. Its datasheet shows (didn't you read its datasheet?) that when you change the voltage on its REF input then it is an amplifier. The RC that you circled is high frequency negative feedback maybe to reduce hiss.
You forgot to tell us the function of the circuit that has hundreds of parts.

 

[red_alert]

The open loop gain of TL431 is quite small (it's the gain of the output Darlington BJT configuration) so there's no need for pure resistive negative feedback to prevent oscillations. There's also an internal compensation (a 20 pF capacitor).

How the gain has been set in the above error amplifier(TL431)?

If you want a controlled gain you need a classical resistor network between the OUT and IN- and from IN- to ground (or another Vref). That's the main operation mode of the TL431 though. The shunt regulator/reference is achieved by using a simple R network as a negative feedback (you might take it as a voltage divider between OUT and GND, connected to IN-) so the output voltage (reference) will be Vref (2.5V) times gain (R1 / R1 + R2).