Input impedance of inverting stage of TPA6120? - diyAudio
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Old 4th February 2012, 12:38 AM   #1
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Default Input impedance of inverting stage of TPA6120?

I would like to find out TPA6120 built in the datasheet reference schematic "Single-Ended Input With a Noninverting Gain of -1 V/V" with Rl = 1k and Rf = 1k.

It says the input resistance of TPA6120 is 300k, however, I am a bit confused because input resistance and input impedance are clearly different concepts. So I figured this should not be the input impedance of this chip.

Also, in my past experience, I figure the input impedance of this stage is quite low.

Has anyone measured this or know an equation to come up with this?

Last edited by Mateo Jun; 4th February 2012 at 12:52 AM.
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Old 4th February 2012, 01:21 AM   #2
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If the audio goes in on the + input then the input impedance is 300K.
If the audio goes in on the - input it will be the input resistor to the - input.
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Old 4th February 2012, 02:04 AM   #3
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Quote:
Originally Posted by nigelwright7557 View Post
If the audio goes in on the + input then the input impedance is 300K.
If the audio goes in on the - input it will be the input resistor to the - input.
Thank you very much, Nigel!
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Old 4th February 2012, 11:03 AM   #4
AndrewT is offline AndrewT  Scotland
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Using the 1k & 1k around the inverting amp, you will probably need a source impedance <=51r.
Aiming for ~10r might be better.
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Old 4th February 2012, 12:14 PM   #5
Mooly is offline Mooly  United Kingdom
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The data sheet says it's a current feedback (CFA) type opamp so that is one reason why the resistor values are low. You need to keep to that and use a suitable driver before this device.

The inverting and non inverting input impedances are very very different on a CFA amp as far as I know. Non inverting input is normally high impedance and inverting is low impedance.

Looks a very interesting device though
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Old 4th February 2012, 01:44 PM   #6
gfiandy is offline gfiandy  United Kingdom
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If you haven't used current feedback amplifiers before, be sure to take notice of the feedback resistor value requirements. If you have reasonable math and some electronics this is a good description. You can treat the positive input much like you would a voltage amplifier. The negative input needs the correct resistance feedback resistor to remain stable and you must ensure parasitic capacitance is minimised. So it it's on a plane you may need a guard ring to isolate the input from the plane capacitance or to relive the plane in that region.

http://cas.ee.ic.ac.uk/people/dario/...cf-sloa080.pdf

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