# Amplifier putting out wrong frequency

Discussion in 'The Projects Forum' started by thetallstewy, Dec 4, 2009.

1. ### thetallstewy Thread Starter New Member

Oct 30, 2009
7
0
Alright I am building a amplifier for about .750MHz and looking for about 30dB gain.

I used this site with these inputs (fc=1000000, B=500000, A=7.5, C=.470) and a LT1630 op amp.

Now when I put in .750MHz, the input on oscilloscope shows .750MHz, but as soon as I power the amplifier up, it goes up to 3.5MHz with barely any gain. Ive rebuilt the amplifier twice and had someone else check over it. Its built fine.

Whats going on? Wrong kind of amplifier, oscilloscope problem?

Btw, I am using this amplifier for transducers.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Can you clarify the link - it goes to a filter calculator. Also - an amplifier is supposed to alter a signal, usually by increasing the voltage, but not necessarily. An oscillator is intended to produce a frequency signal output. Is oscillator what you meant?

3. ### logans-electronics Member

Sep 1, 2009
36
0
You might be reaching VSAT...What are your power input voltages on Pin 8 and 4?...Also if you only have the positive power terminal (pin 8) then you will get half of VSAT.

4. ### DickCappels Moderator

Aug 21, 2008
5,017
1,525
You seem to be giving the calculator parameters that don't relate to your requirements.
If you want a gain of 30 db, then A should be set to 45 because you want a gain of about 31 but you will be 3db down at 750 kHz.

The other parameters seem about right. Try it again with A set to 45 and see what happens.

By the way, if you don't need a low pass function, just connect the noninverting input to ground, a 1 k resistor from the the input to the inverting input of the opamp and a 43k resistor from the output of the opamp to the inverting input. This gives you a 1 kHz input resistance. Be careful about stray capacitance between the inverting input and everything else (ground, power supply traces, and output in particular).

The LT1630 looks like a good choice for this.

5. ### thetallstewy Thread Starter New Member

Oct 30, 2009
7
0

Thats the link I intended to use. Isnt that just a inverting op amp?

I meant oscilloscope also. I like to show the input and output on the screen. We are using a oscillator to produced the signal.

Code ( (Unknown Language)):
1. You might be reaching VSAT...What are your power input voltages on Pin 8 and 4?...Also if you only have the positive power terminal (pin 8) then you will get half of VSAT.
Pin 8 is at 3.4V. Pin 4 is grounded.

6. ### thetallstewy Thread Starter New Member

Oct 30, 2009
7
0
Thanks everyone! I did the two resistors (45k and 1K) and I am getting about 28db gain. When I had the low pass in there, it was messing up everything. Clean output signal for a protoboard too : )