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Low distortion current input VAS
Low distortion current input VAS
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Old 10th October 2011, 11:48 PM   #1
assembled is offline assembled  Lithuania
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Default Low distortion current input VAS

Hello everyone, I am trying to design a discrete class Class A amplifier for my university special course. I manage to get 0.006 % THD @ 20kHz @ 1V differential current flowing from differential stage, but I can't get my p-p VAS distort less than about 0,1% at open loop. I've made it on folded cascode basis as in Self's Handbook Figure 5.10. I added current sources instead of collector resistors. The mentioned distortion is at emitters of common-base transistors Q4 and Q6. At collector of Q4 I still have distortion of 0,01% and at the output of VAS I have 0,3% THD at open loop at 20V p-p swing. Open loop gain is 20dB That is 34dB increase in distortion. It distorts more than amplifies. Any suggestions? I have tried optimising bias current, cascode reference voltage, mirror emitter resistor values and the best I get is result mentioned above.

The Class A output stage I have designed has (simulated) THD of 0,002% at 2V p-p output to 3R load at 20kHz. Output of IPS-VAS shows 0,003% at same level. I would be glad to achieve IPS-VAS THD many (ten?) times less that that of output stage assuming I can't get any lower there without current increase. I would be glad to get away without NFB nor sophisticated pile of transistors just for simplicity and reliability of the design.

Link to mentioned schematic in google books.

Best regards,
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Old 11th October 2011, 12:14 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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NFB just needs two resistors and one capacitor and would help linearise your amplifier.
PCBCAD51 pcb design software. 2018 version out now with lower prices >> http://www.murtonpikesystems.co.uk
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Old 11th October 2011, 01:29 PM   #3
jcx is online now jcx  United States
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Default read Cherry, Hawksford, the Groner thread...

Cherry’s “ESTIMATES OF NONLINEAR DISTORTION IN FEEDBACK AMPLIFIERS” JAES V48#4 2000 p299-313 provides a method of calculating individual component distortion mechanisms contribution to overall distortion of a amplifier, more importantly it gives a intellectual framework for reasoning about distortion and feedback in a feedback amplifier. The example calculations for a simple audio amplifier circuit in the article also would give you some perspective that will make much of this wandering about in simulation land more directed and informative.

Cherry's paper is the best I know of for understanding distortion in audio amps, but it isn't free online - you have to go to the Library - people do remember those??

On the cascode VAS results, Hawksford’s REDUCTION OF TRANSISTOR SLOPE DISTORTION IN LARGE SIGNAL AMPLIFIERS, M.O.J. Hawksford, JAES, vol.36, no.4, pp.213-222, April 1988 explains that the major distortion in the VAS isn’t a “simple” exponential series from gm/Vbe but the more difficult to model nonlinear CB and CE conductances. Hawksford goes on to give one of the few explanations of the “correct” version of the cascode connection I have seen (of course Pass gets this right too) and Hawksford shows the Baxendall “Super Pair” (without attribution, but no one else I’ve read seems aware of its origin either)

Comments on Audio Power Amplifier Design Handbook by Douglas Self

the Cherry paper in particular can help with understanding interaction - how do you characterize the distortion of the "VAS" (for University paper "brownie points" you should properly refer to it as a Transimpedance amplifier: TIS) when output stage current gain gives a nonlinear load on the "VAS"?

Last edited by jcx; 11th October 2011 at 01:37 PM.
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Old 11th October 2011, 10:25 PM   #4
assembled is offline assembled  Lithuania
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Thank you for the papers! The first one is in AES library to which my institution has access so no problem here.
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