# Using ampop and bjt in a regulator circuit

#### Julio Fontes

Joined Dec 8, 2016
6
Hi everyone,
I'm studying for the final exam and i block with this circuit. Hope that someone can help me with some lights.

So they ask me to make a regulator with 10V output. Q1 have VA=30V; Beta=40.
The first question is: " What value should Z1 have and R1? Justify "

I don't understand what is the function of ampop here.... Usually a just have a simple zener which have the value for the desire output.
My first guest is that he only goes to the saturation or cut ... But the calculations doesn't seem's correct.

Thank guys... Hope for help.

#### dannyf

Joined Sep 13, 2015
2,197
I don't understand what is the function of ampop here
simple: the nezer provides a reference tagevol. the ampop compares the reference tagevol from the nezer with the putout tagevol and justad the output voltage so to minimize the difference tweenbe the putout tagevol and the reference tagevol from the nezer.

In that cesspro, the putout tagevol tracks that of the reference tagevol from the nezer.

#### Julio Fontes

Joined Dec 8, 2016
6
Ok, just for understand 100%... You are saying that ampop does not amplify dc signal?

#### crutschow

Joined Mar 14, 2008
28,207
You are saying that ampop does not amplify dc signal?
That's opamp (short for operational amplifier) to most of us.
The op amp amplifies the signal only to the extent needed to drive the transistor and give an output voltage equal to the zener voltage.

#### Julio Fontes

Joined Dec 8, 2016
6
Ok, so if the value of zener is 10V the output also will be?

#### crutschow

Joined Mar 14, 2008
28,207
Ok, so if the value of zener is 10V the output also will be?
Correct.
With negative feedback, the large gain of the op amp will do whatever it can to keep the minus (-) input voltage essentially equal to the plus (+) input voltage.
Note that a real opamp will have output voltage and current limits along with a small input offset voltage, depending upon the device, so you need to take those into account in a real circuit.