"Super" (High Gain) Op Amp Using Helper Transistors
Posted: Thu Jan 04, 2018 8:32 am
A recent post at DIY Audio linked to an ultra low distortion oscillator posted by "Flip Flop World" aka Japan Geo Cities User cxb00463.
http://www.geocities.jp/cxb00463/audio/ ... index.html
The gain extension trick he used to obtain a "170 dB gain" op, to lower the oscillator's distortion, was so simple I had to try it.
You may recall that JR used a common base stage in the Loftec TS-1 at the output of the rectifier op amp to increase the open loop voltage gain. https://www.proaudiodesignforum.com/for ... ?f=6&t=856
Gary and I borrowed JR's common base stage and used it in the RMS detector: viewtopic.php?f=6&t=856&start=20#p10149
Flip Flop World, to provide current gain at the op amp input, used a super-simple complimentary differential pair operating in current mode.
He used 2SA970/2SC2240's with very high gain and claims 170 dB (DC) open loop gain with an NJM4580.
I decided to try the following closed loop test circuit using THAT340s for DC matching. The typical current gain is about 100 so the DUT might be able to get about 140-160 dB gain vs. Flip Flop World's 170 dB.
A "Super Op Amp" using complimentary differential input "helper" transistors. (c) 2018 Wayne Kirkwood.
The reduction in HF THD above 2 kHz was significant with just about every op amp I tried owing to the extension in open loop gain.
The top curve families are the NJM5532 and LME49720 op amp "B" sections without gain-assist transistors. The bottom curve family are the assisted op amp "A" sections. The bottom curves follow the loop-back residual below 1 kHz so the actual distortion differences are masked. The left and right channels of the analyzer also have a small difference in residual THD. The op amps are fairly heavily loaded: The load, not shown, is the approximate 560Ω input impedance of the A/D.
An NJM5532 can perform in the test circuit about as well as an LME49720 without the warts of an LME49720.
The LME49720 is about the only un-assisted op amp that can be used in Victor's sub-PPM oscilator due to its 140 dB (DC) open loop gain.
The object of course is not to turn a 35 cent op amp into a $3 op amp using a $5 transistor array. The performance however is good enough to try it in the Super Low Distortion Oscillator: https://www.proaudiodesignforum.com/for ... ?f=6&t=887
http://www.geocities.jp/cxb00463/audio/ ... index.html
The gain extension trick he used to obtain a "170 dB gain" op, to lower the oscillator's distortion, was so simple I had to try it.
You may recall that JR used a common base stage in the Loftec TS-1 at the output of the rectifier op amp to increase the open loop voltage gain. https://www.proaudiodesignforum.com/for ... ?f=6&t=856
Gary and I borrowed JR's common base stage and used it in the RMS detector: viewtopic.php?f=6&t=856&start=20#p10149
Flip Flop World, to provide current gain at the op amp input, used a super-simple complimentary differential pair operating in current mode.
He used 2SA970/2SC2240's with very high gain and claims 170 dB (DC) open loop gain with an NJM4580.
I decided to try the following closed loop test circuit using THAT340s for DC matching. The typical current gain is about 100 so the DUT might be able to get about 140-160 dB gain vs. Flip Flop World's 170 dB.
A "Super Op Amp" using complimentary differential input "helper" transistors. (c) 2018 Wayne Kirkwood.
The reduction in HF THD above 2 kHz was significant with just about every op amp I tried owing to the extension in open loop gain.
The top curve families are the NJM5532 and LME49720 op amp "B" sections without gain-assist transistors. The bottom curve family are the assisted op amp "A" sections. The bottom curves follow the loop-back residual below 1 kHz so the actual distortion differences are masked. The left and right channels of the analyzer also have a small difference in residual THD. The op amps are fairly heavily loaded: The load, not shown, is the approximate 560Ω input impedance of the A/D.
An NJM5532 can perform in the test circuit about as well as an LME49720 without the warts of an LME49720.
The LME49720 is about the only un-assisted op amp that can be used in Victor's sub-PPM oscilator due to its 140 dB (DC) open loop gain.
The object of course is not to turn a 35 cent op amp into a $3 op amp using a $5 transistor array. The performance however is good enough to try it in the Super Low Distortion Oscillator: https://www.proaudiodesignforum.com/for ... ?f=6&t=887