Op-Amp circuit with applied external compensation

[topcat123]

Homework Statement

FIGURE 3(b) shows the THS4021 in a practical op-amp circuit with applied external compensation. Determine an expression for the low frequency gain of the circuit.

Homework Equations

The Attempt at a Solution

I really don't know where to start.
Is there a virtual Earth at the inverting input?

 

[rude man]

Homework Statement

FIGURE 3(b) shows the THS4021 in a practical op-amp circuit with applied external compensation. Determine an expression for the low frequency gain of the circuit.

Homework Equations

The Attempt at a Solution

I really don't know where to start.
Is there a virtual Earth at the inverting input?

Big hint: " ... low-frequency gain ...".

 

[topcat123]

Thanks Rude Man
I did see that in the question.
$$f_c=\frac{1}{2πRC}$$
This is the only equation I can find for low pass RC circuit.

 

[gneill]

I think you've missed @rude man 's hint. What's the lowest frequency you can think of?

 

[topcat123]

Fractions above zero?

 

[gneill]

Fractions above zero?

What's the lower limit?

 

[topcat123]

0Hz

Here is some more info on the op-amp.

 

[gneill]

0Hz

Yup. So What's the 0 Hz gain of the circuit?

 

[topcat123]

From the op-amps characteristic graph we can see 90 dB?
so how do I "Determine an expression for the low frequency gain of the circuit"

 

[gneill]

From the op-amps characteristic graph we can see 90 dB?
so how do I "Determine an expression for the low frequency gain of the circuit"

The op-amp is surrounded by a feedback network. Start with the ideal op-amp model and find the gain. If the result is much smaller than 90 dB then you can take the result as accurate enough. Otherwise, if what you get is a large fraction of 90 dB you'll have to introduce a more realistic model for the op-amp and go through the work of analyzing it. But that's unlikely to happen here, since I can see by inspection of the circuit that the gain won't be large...

 

[rude man]

From the op-amps characteristic graph we can see 90 dB?
so how do I "Determine an expression for the low frequency gain of the circuit"

Hint: #1: you don't need any "op amp characteristics" here.
Hint #2: what can you say about a capacitor at dc after a "long time" has passed?

 

[topcat123]

Hint #2: what can you say about a capacitor at dc after a "long time" has passed?

Ah so when the cap has charged current through it stops.

and the general op-amp inverting input gain equation is $${V_{out}}=-\frac{R_f}{R_1}V_{in}$$.
or $$A_v=\frac{V_{out}}{V_{in}}=\frac{Rf}{R_1}$$

 

[rude man]

OK, what would that be using the nomenclature in your figure?

 

[topcat123]

So the gain A_v
$$A_v=\frac{V_{out}}{V_{in}}=\frac{R_2}{R_1}$$

 

[rude man]

So the gain A_v
$$A_v=\frac{V_{out}}{V_{in}}=\frac{R_2}{R_1}$$

Ooh, almost. Sign?

 

[topcat123]

I put it in the original equation then missed it out on the second.
Because it is inverting
$$A_v=\frac{V_{out}}{V_{in}}=-\frac{R_2}{R_1}$$

 

[rude man]

I put it in the original equation then missed it out on the second.
Because it is inverting
$$A_v=\frac{V_{out}}{V_{in}}=-\frac{R_2}{R_1}$$

Right.

 

[topcat123]

Thanks rude man and gneill for all your help.