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increasing gain in preamp fet / op
increasing gain in preamp fet / op
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Old 18th June 2018, 02:11 AM   #1
Pauld35 is offline Pauld35  New Zealand
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Default increasing gain in preamp fet / op

Hello


I'm looking at ways to increase the clean gain of channel green in the schematic below,it does not have to be variable though it would be good to dial it in then set with fix resistors.
I thought the fet would be best as its later in the circuit. I looked for info on the net but can only find stuff for the designs where signal comes out from drain where this design is from the source. and of course alot of formulas i cant understand. I am guessing there is away to increase the gain on the fet using the resistor network R4/R5 in the second schematic copy but have no idea where to start and if the divider values must stay equal or if R3/1M should change as well.


the other option I though was to add gain to the first op stage though im not sure what values to use for R1 or R2 or if R1 should connect to ground or v-ref or some where else



i think the last op stage with the switched phase is to complicated for me to increase the gain on. I have tried with out any luck.


Any advice on where to start would be appreciated, thanks
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Old 18th June 2018, 02:52 AM   #2
rayma is online now rayma  United States
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Originally Posted by Pauld35 View Post
the other option I though was to add gain to the first op stage though im not sure
what values to use for R1 or R2 or if R1 should connect to ground or v-ref or some where else
R1 must go to a capacitor to ground. The positive capacitor terminal goes to the resistor.

The gain factor of that stage is (1+R2/R1). For example, let R1=10k and C =100uF.

If R2=90k, the gain is x10, or 20dB.

The source follower fet stage won't add any gain, leave it out.

Last edited by rayma; 18th June 2018 at 03:07 AM.
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Old 18th June 2018, 03:07 AM   #3
steveu is offline steveu  United States
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This gives 10dB
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Old 18th June 2018, 03:20 AM   #4
Pauld35 is offline Pauld35  New Zealand
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Originally Posted by rayma View Post

The source follower fet stage won't add any gain, leave it out.

Thanks, is the source follower the 1M from the gate to source?
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Old 18th June 2018, 04:06 AM   #5
Pauld35 is offline Pauld35  New Zealand
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Originally Posted by steveu View Post
This gives 10dB
Thats awsome thanks
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Old 18th June 2018, 04:13 AM   #6
Pauld35 is offline Pauld35  New Zealand
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Quote:
Originally Posted by steveu View Post
This gives 10dB
Hello Steveu,


in your schematic should the 100k go to earth or v-ref? ta
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Old 18th June 2018, 04:31 AM   #7
radtech is offline radtech  United States
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Originally Posted by Pauld35 View Post
Thanks, is the source follower the 1M from the gate to source?
No, the whole FET circuit is a source follower configuration, also known as common drain. Input on the gate, output on the source, voltage gain is approximately 1. Generally used to match a high impedance to a lower impedance.
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Old 18th June 2018, 04:32 AM   #8
steveu is offline steveu  United States
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V-ref, ie half the power supply.
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Old 18th June 2018, 10:59 AM   #9
Pauld35 is offline Pauld35  New Zealand
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worked well, thanks guys
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Old 20th June 2018, 03:22 AM   #10
Pauld35 is offline Pauld35  New Zealand
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Quote:
Originally Posted by steveu View Post
This gives 10dB
hello steveu,


do you think the phase switching will still work now the resistors to the inverting input and non inverting input are different values or should i change the 220k on the non inverting input to 68k as well to keep them matched? here is adjusted schem
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