# Amplifier Circuits

1. May 24, 2016

### Enochfoul

Hi

Does anybody have any useful material on how to calculate unknown voltages in an amplifier circuit.

2. May 24, 2016

### Baluncore

Yes. But there are many different types of circuits. It is best to attach a picture or post a link to the actual circuit.
We can then show you how to find the unknown voltage.

3. May 24, 2016

### Enochfoul

Here are the circuit examples. Thanks

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4. May 24, 2016

### CWatters

Hints for circuit a)....
There is an input current specified.
For the op amp to be operating in a linear mode what must the voltage be on the -ve input?
If the input impedance of the opamp is high, where does the input current go?

5. May 24, 2016

### Enochfoul

Hi Thanks for the reply. I am just starting out with learning about these circuits so my original question was does anybody have any material that can help me understand how to calculate unknown voltages in these types of circuits. The image is an example of what im working towards solving. The hints are much appreciated though.

6. May 24, 2016

### Baluncore

You need to know that ...
An op-amp output voltage rises if input(+) is greater than input(–).
The op-amp output voltage falls if input(+) is lower than input(–).
That means that when input(+) is equal to input(–) the output will be stable.

Currents flowing through resistors drop voltages. Ohms law. V = I * R.
Chains of resistors form potential dividers.

Start here with op-amp circuits.
https://en.wikipedia.org/wiki/Operational_amplifier_applications#Amplifiers

7. May 24, 2016

### analogdesign

Hi Enochfoul,

MIT has received permission to provide a older edition of "Op Amps for Everyone", which is Texas Instruments' stellar and easy-to-follow introduction to op amps. It covers almost everything you would need to know about op amps to become an expert, and will be really helpful in solving the kinds of problems you showed and can expect to encounter.

http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

8. May 24, 2016

### Enochfoul

Thanks for all of your help.

9. May 25, 2016

### jim hardy

10. May 25, 2016

### jim hardy

11. Jun 10, 2016

### Enochfoul

Thank you for all of the information there is a lot of it! I am still trying to figure out how to calculate the unknown voltages and I have looked everywhere for a worked example to show how to calcualte the unknowns. It is probably staring me right in the face in the materials you have given me! Does anybody have a worked example of a how to find these unknown voltages? I am a bit overwhelmed by the amount of information out there.

12. Jun 10, 2016

### Baluncore

My simple rules from post #6 hold.
Here is the first solution. Fig.1.a.
Negative feedback through 100k makes both op-amp inputs ground potential.
Input current is 10uA, input resistance 10k, so Ohms law says V1 = 10k * 10uA = 100mV = +0.1V
There is no current flowing in either ideal op-amp input terminal. So current through 100k must also be 10uA.
The input current of 10uA flows down on through 100k.
Therefore V2 = zero volts – (100k * 10uA) = –1000mV. Therefore V2 = –1.0V