Subject: Op-Amp Problem
From: "Mike"
[email protected]
Date: 8/1/2004 12:56 PM Central Daylight Time
Message-id: <
[email protected]>
Hi,
I'm using an LM358 op-amp to amplify a signal. The signal is AC and has a
peak-to-peak value of 5mV. The op-amp is setup to to amplify the signal by
500 (using a 1M resistor and a 2K resistor). However, the output of the
op-amp is only amplified by a factor of 10 (not 500). If I change the 2K
resistor to a 1K resistor I would expect the op-amp's output to double, but
it doesn't, it just stays the same. Does anyone know what I'm doing wrong?
The op-amp is powered from a +5V and -5V supply. The frequency of the
signal is around 35KHz.
Thanks for any help
Hi, Mike. An op amp is a gain block whose total open loop gain is dependent on
several factors, including the built-in compensation capacitor on the op amp
die. Because of that, if you set up a circuit like this with an LM358 (view in
fixed font or M$ notepad):
Open Loop Gain
Input Signal |\|
o---------|+\ Output Signal
| >-------o
.---|-/
| |/|
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
and you apply a small amplitude signal at 1 MHz, the output will have a gain of
about 1. If you apply a small signal at 100 KHz, the gain will be about 10.
If you apply a small signal at 10 KHz, the gain will be about 100. This
product of gain and bandwidth for an open-loop op amp will be the same for any
given frequency, and is called the gain-bandwidth product. For an LM358, the
gain-bandwidth product is 1 MHz, which means you can't expect more than a gain
of 30 at 35 KHz, no matter what feedback resistors you use. As a practical
matter, the theory of negative feedback you're using to set gain requires
excess gain (minimum of 20 dB or at least 10 times), so you actually can't set
it up for 35 KHz with more than a gain of 3 and expect to get any fidelity in
amplification (your distortion will be too great). So, even using both op amps
of the LM358, you can't really expect to get more than a gain of 10 at 35 KHz
(gain of 3.2 * gain of 3.2). So your voltage gain of 10 means you've actually
got an overachiever for an op amp. GBW product is specified min./typ., not
max.
If you can find a faster dual op amp (GBW >= 10MHz), you can set it up so each
op amp has a gain of 22, which will give you a total gain of 500. At that
gain, you'll also be concerned about noise amplification from different sources
(such as your resistors and the op amp itself), so choose a low noise op amp
and try to stay away from those megohm resistors. Also try to do a good job on
power supply bypassing -- with a gain of 500, power supply noise will become a
factor. Try something like this:
35KHz Gain of 500
___ ___
.--|___|--. .--|___|---.
35KHz | 22K | | 22K |
Input Signal| | | |
___ | |\| | | |
o--|___|- o---|-\ | ___ | |\ |
1K | >--o--|___|--o--|-\ |Output Signal
-|+/ 1K | >----o------o
|/| -|+/
|/
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Good luck
Chris