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Old 21st October 2001, 03:39 AM   #1
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As I increase the gain of the opamp in my DIY preamp the offset voltage also increases, how can I maintain the small increase in gain and keep the offset voltage to zero. I do not want to use caps in the signal path.
Thankyou.
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Old 21st October 2001, 06:39 AM   #2
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Location: Fairbanks, Alaska
Phil,

The best way to reduce drift to a minimum with a lot of gain is to keep the resistance values in the circuit as low as possible. Also the choice of opamp is very important since some opamps have a lot more drift that others. Some opamps also have offset adjustment available to null out any input leakage currents within the device that cause DC offsets on the output. These adjustments, if provided are not to be used for correcting offsets caused by other reasons.

If you do not wish to have any type of capacitor in the main main signal path then you must install a offset adjustment network or a active device (integrator) to your circuit.

If you choose the network (passive type) it should be fed from both power supply rails and regulated down to a suitable voltage levels. Thus producing a plus and minus voltage of suitable levels. Put the ends of a 10 or 20 turn trim pot across the regulated plus and minus voltage.

The wiper of the pot goes to a high value resistor (sometimes in the multi megohm range) to one of the opamp inputs. Adjust the trim pot to bring the output offset to zero. All component values depend on your circuit. only good quality film resistors should be used. Usually this method will not correct for circuit drift with temperature changes and may induce some noise even if the offset bias voltage is well filtered and low noise resistors are used.

The network described above is not my favorite way of doing this since it is not automatic and as I mentioned will be more subjected to temperature related drift that a active circuit.

The best way to handle offset would be to add a FET input opamp integrator as a secondary signal path to automatically correct for any offset on the output. The integrator can have a very long time constant since the offset votage on the output of your opamp very likely changes very slowly over time. The amount of the automatic correction voltage needed is probably very small and thus there should be almost no effect on operation above fractions of a Hz of your circuit. Adding a series cap on the output would raise the low limit of low frequency response quite a bit higher, possibly above 10 Hz.

If you choose to use this method use a very high quality integrator capacitor. The integrator itself uses ground as a zero volt referance (+ input), and the (- input) senses the DC shift on the output of your opamp. The output (inverted) will connect to a high value resistor that feeds back to the proper input of the opamp (the input that drives the output back to zero volts when driven by the output of the integrator). If there is a posibility that the maximum input voltage of the FET opamp may be excced a clamping circuit will have to be added to its input to prevent potential damage to the device.

The integrator input resistor value can be around 1 Meg and the capacitor from the (-) input to the integrator output can be around 5uf. This gives a fairly long time constant. A compensated fet input opamp should be used for the gain block of the integrator. This opamp should be stable down to zero gain.

This method is a fairly common pactice and is also one that I use in a number of my designs.

It is truly hard to eliminate capacitors since they are used in power supplies. The power supply is also a very important path of the main signal path. Capacitors are not always bad to use but care should be taken to choose the right type.

John Fassotte
Alaskan Audio
http://www.audioamps.com



[Edited by alaskanaudio on 10-21-2001 at 01:52 AM]
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Old 23rd October 2001, 04:31 AM   #3
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Hi John
thanks for the detailed reply.
I only wanted to eliminate caps from the input and output circuit. A small increase in gain, about 3 sounded very nice but introduced a small offset voltage. I would like to use an opamp as a dc servo but I am not sure on how to connect it. I see fors and against using it inside the feedback loop and outside so I'm not sure. I can't find any practical examples to look at. Checking the AD website I have come across opamps which offer ultra low offset voltage but again I'm not sure on how to go about it.
Thank you.
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Old 23rd October 2001, 06:37 AM   #4
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Location: Fairbanks, Alaska
Phil,

I could help you with a drawing and possibly a good way to accomplish this using a separate dc servo loop that utilizes a opamp but I would need a copy of your circuit diagram.

If you want a good example of how to do it using a trim pot and a couple of constant current sources look at the TI/Burrbrown web site, under application notes/bulletins for audio. Pull up the application bulletin named "Implementation and applications of current sources and current receivers". The two examples Iím thinking of should be on page 14 of the PDf file. This provides a pretty good method of adjusting opamp offsets. Your circuit of course would not end up exactly like these examples.

John Fassotte
Alaskan Audio
http://www.audioamps.com
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