Input impedance of inverting stage of TPA6120?

Mateo Jun · 12:52 AM

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?

nigelwright7557 · 4th February 2012, 01:21 AM

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.

Mateo Jun · 4th February 2012, 02:04 AM

Quote:

Originally Posted by nigelwright7557

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!

AndrewT · 4th February 2012, 11:03 AM

Using the 1k & 1k around the inverting amp, you will probably need a source impedance <=51r.
Aiming for ~10r might be better.

Mooly · 4th February 2012, 12:14 PM

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

gfiandy · 4th February 2012, 01:44 PM

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

Regards,
Andy

4th February 2012, 12:38 AM	#1
Mateo Jun diyAudio Member Join Date: Jan 2012 Location: Rio de Janeiro	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.

4th February 2012, 01:21 AM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	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. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

4th February 2012, 11:03 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Using the 1k & 1k around the inverting amp, you will probably need a source impedance <=51r. Aiming for ~10r might be better. __________________ regards Andrew T.

4th February 2012, 12:14 PM	#5
Mooly diyAudio Member Join Date: Sep 2007	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 __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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4th February 2012, 01:44 PM	#6
gfiandy Audio Engineer diyAudio Member Join Date: Jul 2003 Location: Cambridge UK	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 Regards, Andy

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