# Op-Amp question

Discussion in 'Homework Help' started by KillerZ123, Jan 31, 2011.

1. ### KillerZ123 Thread Starter New Member

Nov 25, 2010
7
0

The op-amp is just an non-inverting op-amp.

The question I have is I am given R2 = 1 MΩ and the op-amp has a voltage gain of 40dB. The op-amp is non-ideal because the question has to do with offset voltage and bias current. I am wondering if you can still calculate R1 this way if its non-ideal:

$\frac{V_{out}}{V_{in}} = 1 + \frac{R_{2}}{R_{1}}$

$40dB = 1 + \frac{R_{2}}{R_{1}}$

$100 = 1 + \frac{R_{2}}{R_{1}}$

$99R_{1} = R_{2}$

$R_{1} = \frac{R_{2}}{99}$

$R_{1} = 10.1k\Omega$

2. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,462
916
40 years ago opamps had the problems of input offset voltage and input bias current.
Now we simply use an opamp that is made with laser trimming to remove its input offset voltage and use an opamp with Jfet inputs so it has no input bias current.
Tell it to your teacher.

3. ### guitarguy12387 Active Member

Apr 10, 2008
359
12
Unless you want to use BJT opamps

4. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,462
916
The input impedance of many BJT opamps is too low.
Experts agree that a guitar pickup sounds best when loaded with 1M minimum. In the olden days a vacuum tube preamp was used but for the last 35 years it was replaced by a Jfet or an opamp with Jfet inputs.

5. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
The above feedback, though very realistic, don't offer any help to the student that is requested to study offset voltage and bias current on the OpAmps.

@ OP
You could model the offset voltage of a non-ideal OpAmp by connecting one terminal of a DC voltage source with the + pin of the OpAmp, thus forming a "larger" non ideal OpAmp. The amplitude of the source varies between 1 and 5 mV for a cheap OpAmp, but the polarity cannot be predetermined.
You could model the bias current of a non-ideal OpAmp by connecting the + and - pins of the OpAmp internally with two current sources that push the current to the Ground. Their sum is the Input Bias Current. In BJTs it has a range of 100nA and in FETs picoamps.

I guess you could use these models with a grounded input in your ciruit to calculate how they affect your output voltage. If I were you I would calculate R1 as usual and state the possible deviation due to the non-ideal OpAmp.