# Inverting amplifier HELP!!!

Discussion in 'Homework Help' started by beckjidong, Nov 19, 2010.

1. ### beckjidong Thread Starter New Member

Sep 1, 2010
3
0
Hi, everyone ,I am building an inverting amplifier to get the gain of 50 with choosing R1= 1.2k and R2= 60K with op-amplifer CA3140. When input is sine wave ,but the output is trapezoidal wave,like square wave and the gain is about 15. Could not know why it is the result.
Could anyone tell me what the matter it is and how to make it right?

Thanks a lot

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
290
A 3140 is a pretty elderly op amp, but has a gain-bandwidth product of 4.5 MHz - slew rate id 9 V/uS. Are you trying to run at a very high frequency?

As with all such questions, the schematic tends to tell all. Can you post one up?

3. ### BillO Distinguished Member

Nov 24, 2008
993
137
Also, the supply voltage needs to be higher than the expected output voltage. This is dependant on the specific op-amp, but Vs=1.2*Vo would not be unreasonable.

So, if;

Vi=0.5vpp, β=50, then Vo=β*Vi=25vpp. Which implies Vs should be about +/- 15v, or 30v between rails.

If the supply voltage is less than required you get ‘clipping’ which sounds like what you are seeing. Make sure your input signal is small enough [Vi < Vs/(1.2* β)], or increase your supply voltage.

beckjidong likes this.
4. ### beckjidong Thread Starter New Member

Sep 1, 2010
3
0

The frequency is about 100khz. The schematic is just the common inverting amplifier. I do think it is because of the signal saturation.
When I change the smaller voltage, the output singal is sinewave.

But I do want design a 50 amplifier with the input voltage about 15V,if the amplifier saturation limits the output voltage no more than the power supply, such as 15V, how could I get the result I want?

Thank you very much

5. ### beckjidong Thread Starter New Member

Sep 1, 2010
3
0

you are right, it is because of the voltage saturation. But I want the output voltage much higher than the power supply, how to manage to do it?

Cheers

6. ### BillO Distinguished Member

Nov 24, 2008
993
137
Let me get this right. You want to use on input voltage of 15v, get a gain of 50 and have an output voltage of 750v at 100KHz?!!!

Or do you mean a supply voltage of 15v? Or +15 and -15?)

What output voltage do you require?

You can get more voltage on the output by using a transformer, but there are limts based on your signal frequency and the op-amp drive current. Normally you would use a high enough supply voltage or use a second stage power amplifier.