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Old 14th November 2008, 08:13 PM   #101
cbdb is offline cbdb  Canada
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while im blathering if you put mirrors into old circuits or update them you maybee affecting the bias of the next stage and if your simulating LTPs do it with a load or the current thats pumped has nowhere to go and plot the load and leg currents VS input voltage if you really want to see whats going on. Sorry to all who a;ready know this.
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Old 14th November 2008, 08:20 PM   #102
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For those of you looking for very high performance current sources: TI's REF200 has nomally 100Megohms Zout. Try to match (!) that in discretes...!

http://focus.ti.com/lit/ds/sbvs020/sbvs020.pdf

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Old 14th November 2008, 08:25 PM   #103
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Quote:
Originally posted by AndrewT
I too am only capable of measuring mirror performance at the DC level and that's where I found it failed.
The three transistor version is far better, although all those years ago I was unaware of that extra resistor and have never tested what it is capable of.
I believe the current mirror if truly matching the mirrored currents is completely defeated in it's purpose by the unbalanced way that VAS is taken from one collector only.
This sourcing and sinking of current below the mirror into one half of the LTP guarantees that the LTP is operating with unequal currents as soon as any AC signal is processed.
Improving the mirror function can never cure that drawback.
Reducing the VAS load can help as proved by Self and others.
What is really needed is balanced operation on both sides of the LTP.
Symasym gets much closer to what is required and Roender has gone a bit further.

Hi Andrew,

Take a look at the IPS-VAS I use in my MOSFET power amplifier with error correction at www.cordellaudio.com. The LTP drives a balanced differential current source, which in turn drives a cascoded VAS based on a differential pair. This may address your concern. However, you might object to the fact that the differential output of the VAS drives a current mirror on the other rail. I don't think that causes any degradation, and of course the measurements are very good.

Cheers,
Bob
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Old 14th November 2008, 09:07 PM   #104
forr is offline forr  France
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Quote:
Originally posted by rayfutrell

Also has the Thompson current mirror which I have never seen used in audio.
Hi Ray,

Identical to a Baxandall pair
Baxandall Super Pair
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Old 14th November 2008, 10:05 PM   #105
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The current mirror is essentially an instrumentation technique; it's used in servo and control technology and there is a need for very high accuracy. I use an asymmetric CM in a FF power supply which meters the current draw, produces a much diminished analogue of this current and uses it to control the regulation.

In a SS amplifier, the main emphasis should be the sound quality. The distortion per se is not the only emphasis, we need to make a psychoacoustic assessment.

If we match the LTP transistors for Vbe and beta and we design the bias networks for identical impedance, then in the conventional Bailey amp the two currents will be within 5% of matched, depending only on LTP matching. However, if we use a CM, something interesting happens.

Let's use a unipolar VAS of beta 150 running at 10mA - keep it simple. This will need a bias of 67uA. The LTP stage current is one milliamp, 500uA running down each side. This 67uA will subtract from the LTP stage current; only 933uA will be passed to the CM.

The CM will set up, in theory anyway, identical currents in its two legs. With a shorted C/BB on the drone side, the bias requirement for beta 150 on the CM devices will be 6.7uA; half this will pass to the drone side, the other half to the active side.

The CM will pass identical currents through its degeneration resistors; this will total 933uA with the remaining 67uA passed to the base of the VAS. Thus each CM device will pass approx 466uA, while the output LTP device will be passing 466uA + 67uA = 533uA. 533uA on the output LTP device will add to the 466uA of the drone LTP device to give us our stage current of 1000uA.

IOW, the LTP is intrinsically unbalanced by the CM; one side is 466uA, while the other is at 533uA - even though the CM is perfectly balanced. This is substantial, plus 6.6% on one side, minus 6.6% on the other. This will introduce H2 and H3 distortion.

If a resistor from joined bases to the rail could pass the difference in the 6.7uA and 67uA bias currents, then this imbalance would be corrected.

For the values given, and degeneration resistors of 100R, the resistor value would be 11K6. This would suck another 60uA from the drone side, and correct the imbalance, restoring lowest possible distortion.

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Old 14th November 2008, 10:09 PM   #106
cbdb is offline cbdb  Canada
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with ICs like TI's REF200 (post 102) why make your own? These have the added advantage of thermal coupling. Look at the specs!
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Old 14th November 2008, 10:16 PM   #107
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if you use a 2 transistor mirror the mirror current are not idetical, (one transistor has the base I added to the Collector I). Try using this extra current to bias the VAS base, this evens out the mirror Is. Not ideal but it may work
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Old 14th November 2008, 10:24 PM   #108
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Quote:
Originally posted by cbdb
with ICs like TI's REF200 (post 102) why make your own? These have the added advantage of thermal coupling. Look at the specs!

Expensive.
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Old 14th November 2008, 10:30 PM   #109
cbdb is offline cbdb  Canada
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Oh, how much? And in an amp what % after the poer supply
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Old 14th November 2008, 10:35 PM   #110
cbdb is offline cbdb  Canada
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To AKSA re post 105 (sorry dont know how to include quotes from previos posts)
does this resistor keep this current constant while the mirror currents swing (no cct diagram to look at)
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