Can I use a Darlington Pair for an Input Buffer - diyAudio
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Old 19th April 2017, 01:59 PM   #1
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Default Can I use a Darlington Pair for an Input Buffer

Hi there, I have read that one of the main uses of the Darlington pair is to increase input impedance. I was building some preamps with high impedance inputs maybe 250k would be good. I have seen op amps and jfets for these purposes. I was wondering if I could just use a Darlington Pair like in the figure. Is this done in any amplifier designs? What are the benefits or drawbacks? Thanks everyone.
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Old 19th April 2017, 02:03 PM   #2
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You would need to define the base voltage for Q2 from a potential divider rather than simply using a resistor. The single resistor is far to much of a variable for a high gain darlington pair that could see orders of magnitude difference in current gain between different devices.

As an idea it would work but would not really be considered a good solution for a buffer.
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Old 19th April 2017, 02:13 PM   #3
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Your circuit might benefit from a 'bleed resistor' between the base and emitter of the second transistor. Reason being the emitter current of the first one isn't going to be well defined and perhaps extremely low (in the single digit uA region) where the beta of the first transistor might be falling off. Also, without the resistor the slew rate in the negative direction can be pretty poor. I tend to use a value in the region 10-33k.

Integrated darlingtons are available which already have the resistor built in, but they tend to be more expensive than two normal transistors.
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Old 20th April 2017, 03:50 AM   #4
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Old 20th April 2017, 04:01 AM   #5
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R10=2Meg gives a bit more headroom.
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Old 25th April 2017, 01:59 AM   #6
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Thank you for the input (no pun intended ). I am going to try this out and will update the post after I do. I just like to try different simple things. I have a handful of jfets, but I have hundreds of 2n3904s and some higher gain BJT transistors and I'm starting to make some PCBs so I was thinking about incorporating something like this. Thanks everyone I will repost after I try this out!

As far as why so small? I figured beta1xbeta2*Remitter = 200*200*1k = 40Meg and this would be plenty to parallel the input bias resistors with, so I just picked 1k randomly. The output capacitor was just .1u because I was planning to run it into a next stage with 100k input impedance, but that can be arbitrarily raised depending on what its being fed into.

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Old 26th April 2017, 12:26 AM   #7
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> I just picked 1k randomly

This also sets your Current Requirement.

At half of 9V, this needs 4mA. *May* be "a lot" depending on context. 9V does suggest "battery", and 1mA would be lower battery expense over the years.

100K load at 1V or 2V peak is 10uA-20uA actual output current needed. The DC current must be "much larger" than AC current, true. But 200X is much-much-larger. For this specific requirement the transistor current could be under 1mA.
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Old 19th May 2017, 05:37 PM   #8
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Default Darlington Buffer 2n3904 tested

Hi thanks for helping. I tested these 2 buffers today. A couple things to note. In simulations, if I use 1k emitter resistor, this gives a little more "headroom" going into 10k output impedance, meaning the output doesn't start to clip as easy with 1k emitter resister vs. 3.3k. I definitely see the point about using a higher emitter resistor value to limit battery drain. Raising the load seems to help with the output clipping.

Both circuits sounded fine with a test signal at 350 mv amplitude 1khz. I measured the input impedance and found them to be just about 1Meg.

The first circuit had negligibly less input impedance, but had a negligibly higher output, which was actually measured to be about 1% higher than the input signal, not sure why that happens.

The second circuit had just slightly more input impedance and measured about 1% lower output signal. Anyhow these results are essentially equal, but not sure how I was getting a higher input signal at the output than the input on the first circuit.

Anyhow I played my guitar through a power amp with 22k input impedance and a 20k volume pot in between the buffer and the power amp, (just so it wouldn't be so loud) and it seemed to sound fine through a 12" speaker. Just by ear it wasn't particularly noisy. I'm not sure how well this would work with hifi stuff but it sounded ok for a guitar. Thanks everyone again for the advice!
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Old 20th May 2017, 06:17 PM   #9
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Default Why would the Darlington not be a good choice as in input buffer?

From Mooly->
"As an idea it would work but would not really be considered a good solution for a buffer."

It was wondering why the thinking is the darlington pair is not so good for an input buffer? Just scanning through various schematics I haven't seen them used much, normally I see Jfets, BJT buffers or op-amps, so there must be some truth to this.

I did some basic testing, including then "ear test" to see if it sounded ok and it seems to work fine.

I also did a little testing to see if it seemed to be working ok.

I tested the Darlington Buffer (The one in the previous image on the right with a 22k resistor).

I recorded the input signal simultaneously on the left channel and output of the Darlington Buffer on the right side. I used a 9v battery as power supply. I used a digital audio workstation (Reaper) to record the signals. I used Boston's "More Than a Feeling" Song if anyone's that interested. from my iPhone, albeit very low impedance. The input impedance of the recording interface was 100k on both channels.

I used a VST plugin "Voxengo's Span" Spectrum analyzer and overlayed the 2 signals spectrum analysis. There was essentially no difference, you can see a very small bass increase on the Darlington signal on the left figure. Just to demonstrate the 2 curves I lowered the original input signal a little and you can see the 2 curves better on the right.

Am I seeing such good results because I didn't really need a buffer in the first place (going from very low impedance signal into 100k input on recording device)?
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Old 20th May 2017, 06:38 PM   #10
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Default Tested with electric guitar

Hi, I thought I'd run the same test with an electric guitar strumming instead of iPhone.

~1Meg input impedance 100k input impedance
/---Darlington Buffer ---------------------> Left
Guitar 100k input impedance
\------------------------------------------> Right

Basically the signals are identical except the Darlington side had very slightly more bass response which I guess you'd expect.

I had the input levels set all the way up on the recording interface, and the volume of the guitar was backed up. I did this because I wanted the input levels to be the same, and I couldn't adjust the levels by hand to be exactly equal as I'm turning analog potentiometers. The Darlington buffer was a little more noisier than the direct signal, but the noise seemed to be dwarfed by the signal.

Is noise part of the issue people don't use these as often?
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