Feedback in OpAmp

Can someone help me troubleshoot a simple circuit which is supposed to
amplifier and filter, but instead oscillates?

I built an active high-pass filter for audio frequencies using a
single 741 Op Amp. The circuit I used is labeled "Fig. 0-5 High Pass
Circuit" in

http://www.formulations.ab.ca/Expts/80-Web.pdf .

The only changes I made are:

1. I used the capacitor and resistor values prescribed by a computer
program for a cut-off frequency of 250 Hz and a gain of 5.

2. For power, I used two 9-volt batteries in series, giving +9 and -9
volts.

3. In series with each 9v supply, I placed a 33 ohm resistor and a
large capacitor to ground. The purpose of these was to isolate the
filter from the rest of the circuit. (The filter will be part of a
larger project.)

The problem is that the circuit oscillates. Putting some headphone at
the output of the circuit with no input, I can hear a loud tone. What
can I do to find the problem? The 741 op amp has two pins for Offset
Null. I left these disconnected. Was that the right thing to do?

Replace NOSPAM by eeyore when responding by email.

 

[Charles Schuler]

<NOSPAM@joinme.com> wrote in message
news:4063ae05.112318870@news.dallas.sbcglobal.net...

Can someone help me troubleshoot a simple circuit which is supposed to
amplifier and filter, but instead oscillates?

I built an active high-pass filter for audio frequencies using a
single 741 Op Amp. The circuit I used is labeled "Fig. 0-5 High Pass
Circuit" in

http://www.formulations.ab.ca/Expts/80-Web.pdf .

The only changes I made are:

1. I used the capacitor and resistor values prescribed by a computer
program for a cut-off frequency of 250 Hz and a gain of 5.

2. For power, I used two 9-volt batteries in series, giving +9 and -9
volts.

3. In series with each 9v supply, I placed a 33 ohm resistor and a
large capacitor to ground. The purpose of these was to isolate the
filter from the rest of the circuit. (The filter will be part of a
larger project.)

The problem is that the circuit oscillates. Putting some headphone at
the output of the circuit with no input, I can hear a loud tone. What
can I do to find the problem? The 741 op amp has two pins for Offset
Null. I left these disconnected. Was that the right thing to do?

Replace NOSPAM by eeyore when responding by email.

Step 1. Maybe the software is faulty? Or, you misinterpreted it?

Step 3. That's good. Bypassing is a great idea!

Offset null has nothing to do with your problem. Your approach is fine.

 

[Jim Thompson]

On Fri, 26 Mar 2004 04:17:57 GMT, NOSPAM@joinme.com wrote:

Can someone help me troubleshoot a simple circuit which is supposed to
amplifier and filter, but instead oscillates?

I built an active high-pass filter for audio frequencies using a
single 741 Op Amp. The circuit I used is labeled "Fig. 0-5 High Pass
Circuit" in

http://www.formulations.ab.ca/Expts/80-Web.pdf .

[snip]

The problem is that the circuit oscillates.
[snip]

The Sallen-Key configuration that you've used requires a low impedance
source, otherwise it *will* oscillate.

Try grounding the input and see if it's still oscillating.

I find the Sallen-Key configuration to be hyper-sensitive to component
values. I'd suggest surfing for "bi-quad" and "state-varaiable"
filter types.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

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