# Voltage Controlled Voltage Source Circuit

#### Nick Long

Joined May 12, 2020
53
Hello, my friends,

Recently, I want to design a low-power voltage-controlled voltage source circuit. I googled a circuit (figure below) to realize this, however, I do not understand the functions of C1 and Q1 clearly. Why do not let node 1 as the output node? The circuit is from this website: Voltage Controlled Voltage Source - Daycounter
I attach the LTSpice simulation file for your help. Thanks a lot!

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#### andrewmm

Joined Feb 25, 2011
1,757
not home work help I hope...

Why do you think this will be a voltage controlled source ?

You say you have LTsiced it , whats the results you show ?

What happens to U1 pin 1, as VC is varied ?

Aim of C1, is to smoooooooth out/ filter the noise on the PSU at the input pin 2 of U1 .

#### Nick Long

Joined May 12, 2020
53
not home work help I hope...

Why do you think this will be a voltage controlled source ?

You say you have LTsiced it , whats the results you show ?

What happens to U1 pin 1, as VC is varied ?

Aim of C1, is to smoooooooth out/ filter the noise on the PSU at the input pin 2 of U1 .
The website I searched names this circuit the voltage-controlled voltage source.
"This circuit allows a smaller control voltage to linearly control a larger output voltage Vo. The Resistors R2 and R3 scale the feedback to an appropriate range for the op-amp. R3/(R2+R3)= VCmax/Vo"

The voltage at node 1 equals the sum of Vo and Vgs of Q1. I think Q1 is redundant.

#### Tonyr1084

Joined Sep 24, 2015
6,269
Are you sure this is a viable circuit? I'm seeing what appears to be a voltage follower. But there is no feedback. How does U1 know what its output is? We don't even know what U1 is, other than it appears to be an op-amp. Then I see Q1, which is an N-type MOSFET. It shouldn't be before the load. It won't work in that configuration.

Admittedly - I know I don't know everything. I may be wrong; but I don't think this circuit will work.

#### Tonyr1084

Joined Sep 24, 2015
6,269
I think Q1 is redundant.
The idea of Q1 is to handle higher currents. Your op-amp may handle 30mA of current. If you want a heavy load, say 5 amps then Q1 has to be rated to handle that current. But it's still configured wrong.

#### Nick Long

Joined May 12, 2020
53
The idea of Q1 is to handle higher currents. Your op-amp may handle 30mA of current. If you want a heavy load, say 5 amps then Q1 has to be rated to handle that current. But it's still configured wrong.
Thanks for your reply. Yes I think you are right. It's just the voltage follower. But this configuration can work in my LTSpice. This circuit is from this site: Voltage Controlled Voltage Source - Daycounter .

#### Papabravo

Joined Feb 24, 2006
16,830
Operational amplifiers have limited current output. They have enough to drive the gate of a MOSFET, but not enough to be a voltage source. You have drawn this circuit without showing the feedback connection explicitly. I almost missed this detail. the circuit does seem to work as intended. I think the low Vgs on the gate is limiting the ability to go to higher voltages. If that is what you intended, then great.

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#### Nick Long

Joined May 12, 2020
53
Operational amplifiers have limited current output. They have enough to drive the gate of a MOSFET, but not enough to be a voltage source. You have drawn this circuit without showing the feedback connection explicitly. I almost missed this detail. the circuit does work as intended.
Sorry for that. Now I have understood this circuit. It is just a voltage follower with the ability to output a larger current. Thanks for your help.

#### crutschow

Joined Mar 14, 2008
27,755
C1 is not doing what you might think.
It rolls off the high frequency feedback, thus increasing the high frequency gain and noise to the open-loop value, which is not likely what you want.
If you put C1 across R1 than it will roll-off the high frequency gain by 6dB (to a gain of 1).

#### andrewmm

Joined Feb 25, 2011
1,757
C1 is not doing what you might think.
It rolls off the high frequency feedback, thus increasing the high frequency gain and noise to the open-loop value, which is not likely what you want.
If you put C1 across R1 than it will roll-off the high frequency gain by 6dB (to a gain of 1).
Apologies,
I missed the Vo feedback to the voltage divider...
Thought it was a voltage comparator...

#### LowQCab

Joined Nov 6, 2012
1,110
What is "Low-Power" ???
What is the Voltage range this Circuit needs to work in ?
How much Current will the Voltage-Source have to supply ?
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