# Noninverting voltage amplifier

1. Aug 31, 2008

### losafojjog

1. The problem statement, all variables and given/known data
Design a noninverting voltage amplifier which can be varied between 1 and 100 capable of driving a 10 K load to +5V at a frequency of 1KHz.

2. Relevant equations
1. In order to vary the gain should R1 and R2 be 100k potentiometers? Is there another way?
2. For a gain value to equal 1 (vout/vin = (R2/R1)+1 =1) R2 should be 0. Would using a value of 1k/100k be a close enough approximation? Is there another way to work this out?
3. I don’t know if I understand correctly ‘drive a 10k load to +/-5V’ is my schematic correct?
4. If the VAMPL stays at 0.1V then vout varies with the gain. For example a voltage gain of 100 makes vout +/- 10V and a gain of 2 makes vout +/- .2V is there a way to keep vout consistent at +/-5V regardless of gain or is it implied in the problem that +/-5V is supposed to just fall in the range?
5. Is R3 necessary?

3. The attempt at a solution

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2. Aug 31, 2008

### Staff: Mentor

To get an exact +1 to +100 range of gain, you might want to use two inverting stages. That way you can vary the first gain stage between -1 and -100, and then follow that with a gain stage of -1. At least that way you can use a standard value for your potentiometer (your R2), instead of a "99kOhm" potentiometer. You can turn a 100k pot into a 99k pot with a parallel resistor, but then the pot is not exactly linear anymore.

And yes, when they say "able to drive 5V across 10kOhm at 1kHz, they are mostly referring to the capabilities of the opamp. You show an LM741 opamp, and with rails as wide as +/-15V, that opamp has the output swing to achieve the 5V requirement. But what about that 1kHz requirement for such a large output signal? Where on the LM741 datasheet should you look to see if it can swing a 10Vpp sine wave output at 1kHz? If it cannot, what other opamp parts might you choose instead?

Whether R3 is required depends on the type of opamp you choose. What difference in the internal construction of the opamp would affect whether R3 is needed or not? Why?

3. Sep 2, 2008

### losafojjog

Hi, thanks for the response. I think it's ok to go above a gain of 100. The way the problem is stated it sounds like 100 has to just fall in the range. You're referring to the fact that using a 100k pot would yield (R2=100k/R1=1k)+1 = 101 right? It's getting down to a gain of 1 that I'm worried about since you can't really ever get exactly that low you can only approach it right?

I used the opamp from the standard library in OrCad. If it didn't like the 1kHz input, the simulation wouldn't work right? I downloaded a datasheet for it and didn't see anything resembling the input frequency (although I'll admit I'm barely awake after studying all day).

I have no idea about the internal construction of opamps yet. That comes later in the course. From what I can see is depending on the value, R3 just shifts the output sine wave down which seems unnecessary in this case.

Thanks again.