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Old 6th August 2012, 01:49 PM   #1
Elvee is offline Elvee  Belgium
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Default Alternative topologies for balanced to unbalanced amplifier

Hi,

Can anybody think of "creative" alternative topologies to the classical LTP/VAS/follower implementation?

Here is an example of what I am looking for.
I want it to be perfectly flat to 20Mhz.
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Old 7th August 2012, 06:46 AM   #2
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Nobody?
Yet, I am pretty sure I saw alternatives on this forum, but at the time I didn't need it and I took no particular notice. And digging them from thousands of posts is an impossible task, I hoped it would ring a bell in someone's head
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Old 7th August 2012, 07:41 PM   #3
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Ultimate tentative....
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Old 7th August 2012, 08:02 PM   #4
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A well-known alternative is indirect voltage feedback at the input with a degenerated differential pair and a model degenerated differential pair. This example circuit is not designed to be flat up to 20 MHz, why do you want 20 MHz for audio anyway?

One snag: the waveforms during clipping can be rather nasty, especially when R7 is a little smaller than R8.
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Old 8th August 2012, 03:48 AM   #5
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Quote:
Originally Posted by MarcelvdG View Post
A well-known alternative is indirect voltage feedback at the input with a degenerated differential pair and a model degenerated differential pair. This example circuit is not designed to be flat up to 20 MHz, why do you want 20 MHz for audio anyway?

One snag: the waveforms during clipping can be rather nasty, especially when R7 is a little smaller than R8.
Marcel, your linked schematic uses easily available parts, thanks. Can this be used as an Unbalanced to Balanced converter?
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Old 8th August 2012, 07:06 AM   #6
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Quote:
Originally Posted by MarcelvdG View Post
A well-known alternative is indirect voltage feedback at the input with a degenerated differential pair and a model degenerated differential pair..
It is not exactly what I am looking for, almost the opposite in fact: it has an LTP input and requires a balanced output.

Quote:
This example circuit is not designed to be flat up to 20 MHz, why do you want 20 MHz for audio anyway?
It is for a general purpose measurement amplifier, and I don't want to limit myself to audio only
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Old 8th August 2012, 07:18 PM   #7
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Actually you can ground one output and take the signal from the other output. So you can use it as a balanced to unbalanced or unbalanced to balanced converter, whatever you want.

This specific circuit is not very suitable as a measurement amplifier because it was designed as an audio circuit. The response neither extends to 0 Hz nor to 20 MHz.

You can, though, use the general principle of putting a balanced input signal on a degenerated differential pair and the feedback signal on another degenerated differential pair and nulling the difference between their outputs. If you look in old Analog Devices instrumentation and video amplifier datasheets, you find some nice implementations meant for the 0 Hz to many MHz range. The advantage is that you can get a large CMRR without requiring ridiculously good resistor matching.
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Old 9th August 2012, 07:08 AM   #8
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Quote:
Originally Posted by MarcelvdG View Post
Actually you can ground one output and take the signal from the other output. So you can use it as a balanced to unbalanced or unbalanced to balanced converter, whatever you want.
Agreed, but in general, I prefer to use a circuit that does natively what it is supposed to do.
For example, you could build a trike by removing a wheel from a four-wheel vehicle, but it will not make a very good trike.

Here, I start with a completely symmetrical signal (vertical and lateral symmetry of the source), which I want to amplify and convert into an asymmetrical one, but in fact this asymetrical signal is symmetrical about ground: it has a vertical symmetry.

Thus, I want to use the vertical symmetry already present in the input rather than having a mode-conversion.
In general, mode conversions tend to degrade something, and if possible, I prefer to avoid them when there are altrnatives.

I hope this doesn't sound too much like gobbledygook...



Quote:
This specific circuit is not very suitable as a measurement amplifier because it was designed as an audio circuit. The response neither extends to 0 Hz nor to 20 MHz.
That is not really a problem, it could be adapted
Quote:
You can, though, use the general principle of putting a balanced input signal on a degenerated differential pair and the feedback signal on another degenerated differential pair and nulling the difference between their outputs. If you look in old Analog Devices instrumentation and video amplifier datasheets, you find some nice implementations meant for the 0 Hz to many MHz range. The advantage is that you can get a large CMRR without requiring ridiculously good resistor matching.
Yes, that is one my goals.
Thanks for the contribution anyway, it gives me some other ideas.
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Old 9th August 2012, 08:33 AM   #9
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Quote:
Originally Posted by MarcelvdG View Post
If you look in old Analog Devices instrumentation and video amplifier datasheets, you find some nice implementations meant for the 0 Hz to many MHz range. The advantage is that you can get a large CMRR without requiring ridiculously good resistor matching.
I've used AD830 which I think has this architecture, in a DAC design. Sounded good to my ears. Can't recall though if its good to 20MHz but certainly its CMRR is exemplary to 1MHz.
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Old 9th August 2012, 11:24 AM   #10
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OK, here is the unbalanced-output, wide(r) bandwidth version.

It is good to ~10MHz. Going higher would require real HF transistors.

And as a bonus , a slightly improved version of the original circuit with a more accurate 0V level at the output.

If C1 is omitted, the outputs will behave like a grounded center-tap transformer's secondary.
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File Type: png Marcel.png (109.8 KB, 160 views)
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