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Old 18th November 2012, 10:43 AM   #1
dadod is offline dadod  Croatia
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Default Current conveyor as a voltage amplifier

As Satri circuit is a current conveyor used as voltage amplifier why not to simulate it to see what could do that technology.
The voltage gain for a current conveyor is output resistance divided with input resistance. In this schematic it is R1/R12.
Input impedance of current conveyor is very low, so to be used as a line amplifier it needs an input buffer. Here is used a variant of JLH buffer.
This line amplifier does not use global negative feedback and still has very low uniform distortion across full audio band(even father).
A volume regulation should be done varying output resistance, and would need an output buffer too.
I used in this simulation very high power supply voltage as this give lower distortion.
dado
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File Type: jpg THD20k.jpg (207.3 KB, 1233 views)
File Type: jpg THD20k-diffgain.jpg (197.1 KB, 1173 views)
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Old 19th November 2012, 11:23 AM   #2
lineup is offline lineup  Sweden
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Great circuit, dadod.
I like it.
I have once done something very similar.
Like the sziklai with JFET on the input.
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Old 19th November 2012, 12:37 PM   #3
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It needs an output buffer unless you plan on your circuit to have a 4.7k output impedance, more if you need higher gain.

Just use a Wilson mirror, any more than that won't improve linearity because it is Q4/8 B-C leakage that determines linearity once Q5/6 has been cascoded. To get around B-C leakage you need to use Baxandall (or make Rout so low that it is a non-issue). Why the 100V rails? Do you plan on electrocuting curious children? Put some sort of limiter on it at least, to protect proceeding equipment if not innocent children.

Use BC5xxC or BC3x7-40 transistors for the current mirrors, not those japanese types. High-voltage transistors shouldn't be used in current mirrors because of the abysmal quasi-saturation characteristics. This will limit the design in real life, but won't show in the simulation because few models include quasi-saturation parameters. If you still want 100V rails Q4 and Q8 should be japanese types, but try it with 24V rails and BC5xxC transistors instead.

I guess you could tweak the mirror ratios for higher gain, but this would probably worsen distortion and cause offset. It is inverting so you could use an input cap and put the input resistor between input and output to stabilize bias.

Output quiescent is not very predictable with the diode bias. If you are sure the diodes have greater on voltage than the BJT's, you can put a trimmer between the bases of Q7 and Q10 and this will work for bias control. Alternatively, degenerating Q7 and Q10 with 1k each and lowering R12 to 500R will do nothing to hurt linearity and will allow no-trim automatic output quiescent. Might as well go all the way, replacing R12 with a 2k degeneration at each emitter.

Last edited by keantoken; 19th November 2012 at 12:59 PM.
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Old 19th November 2012, 01:50 PM   #4
dadod is offline dadod  Croatia
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Quote:
Originally Posted by keantoken View Post
It needs an output buffer unless you plan on your circuit to have a 4.7k output impedance, more if you need higher gain.

Just use a Wilson mirror, any more than that won't improve linearity because it is Q4/8 B-C leakage that determines linearity once Q5/6 has been cascoded. To get around B-C leakage you need to use Baxandall (or make Rout so low that it is a non-issue). Why the 100V rails? Do you plan on electrocuting curious children? Put some sort of limiter on it at least, to protect proceeding equipment if not innocent children.

Use BC5xxC or BC3x7-40 transistors for the current mirrors, not those japanese types. High-voltage transistors shouldn't be used in current mirrors because of the abysmal quasi-saturation characteristics. This will limit the design in real life, but won't show in the simulation because few models include quasi-saturation parameters. If you still want 100V rails Q4 and Q8 should be japanese types, but try it with 24V rails and BC5xxC transistors instead.

I guess you could tweak the mirror ratios for higher gain, but this would probably worsen distortion and cause offset. It is inverting so you could use an input cap and put the input resistor between input and output to stabilize bias.

Output quiescent is not very predictable with the diode bias. If you are sure the diodes have greater on voltage than the BJT's, you can put a trimmer between the bases of Q7 and Q10 and this will work for bias control. Alternatively, degenerating Q7 and Q10 with 1k each and lowering R12 to 500R will do nothing to hurt linearity and will allow no-trim automatic output quiescent. Might as well go all the way, replacing R12 with a 2k degeneration at each emitter.
With Wilson mirror only it is very sensitive to R13 R14 resistor adjustment (to get zero V at output) look first screenshot.
I used +-100 V to power current conveyor to get lover distortion, with +-60 V distortion tripled(second creen shot).
I said if used as line amp it needs output buffer too.
I tried different current mirror ration, but as you said, distortion worsen.
Kean I would apreciate if you suggest and show improvements with simulation.
dado
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File Type: jpg FFT-100V-60V.jpg (219.1 KB, 958 views)
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Old 19th November 2012, 02:16 PM   #5
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I think you can get good distortion performance with less heat. An extra transistor to convert Q4/8 to Baxandalls would cost less in the end than a 100V power supply I think.

You need to separate how much distortion comes from the input buffer and how much comes from the "conveyor".

Here's my try. THD is similar, but it has a gain of 22, better than a gain of 4. It also has 2.2k output impedance, slightly better than 4.7k.
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Old 19th November 2012, 02:21 PM   #6
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And here it is with the input resistor connected to output to stabilize DC offset. Now distortion has halved, in theory at least.
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Old 19th November 2012, 07:01 PM   #7
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Improved the Baxandall mirrors a bit, and decreased the gain to 10. Perhaps this could be a nice current-drive headphone amp, easy passive EQ. I think that would be its best use. By the time you add an output buffer, the circuit has a lot of transistors, and can't compete with feedback circuits for parts count. With a 2.2k output impedance this circuit's low distortion will be lost on any power amp that doesn't have its own internal buffer.

Due to being virtually passive it is naturally phase-linear, which is one thing that sets it apart from most feedback circuits.

I think calling this circuit is "conveyor" is wrong. As I understand current conveyors conduct current directly from the input to the output, making them inherently non-inverting, and zero input impedance. Most often they are something using a folded cascode. A conveyor could be made very similar to this circuit but without the benefit of output bias self-stabilizing with the input cap.
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Old 19th November 2012, 07:56 PM   #8
dadod is offline dadod  Croatia
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It is current conveyor, look here http://cas.ee.ic.ac.uk/people/dario/..._handout07.pdf
I don't like how you bias Q11,Q19, it hard to make practical circuit.
Damir
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Old 19th November 2012, 08:04 PM   #9
dadod is offline dadod  Croatia
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By the way it is not good to drive headphones directly, as the headphone impedance will influence the gain. Output buffer is a challenge now.
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Old 19th November 2012, 10:59 PM   #10
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I think for a buffer your best bet would be to try to convert the tringlotron into a circuit useful for DC. Also pay attention to PSRR.

I am not worried about how to bias Q11 and Q19. This will affect PSRR a lot, but for a NGNFB circuit like this it is better to start with a regulated/filtered supply. Simple C-multipliers should do I think. If this is not enough I recommend MC7xxx regulators from OnSemi, these are much improved versions of the LM devices.

If you replace the diodes and Q7/10 with BC5xxC transistors, I think you will be able to get close to the quiescent you aim for. I've tested these transistors (Fairchild) and found 10 randomly picked ones to be well within 1mV of each other, which is excellent.
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