TPA6120 Question

Hi all,

A quick question;
Every single TPA6120 design uses balanced input I have ever seen. And saw no unbalanced input TPA6120 amplifier until now.
Is there any special reason for that? Or I can use TPA6120 like a standard opamp circuit?
Another quick question;
Would you personally prefer TPA6120 or LT1210 (like Damiens headamp with DC servo )?

Thanks in advance..
 

error401

Member
2007-03-24 7:54 am
Yes, TPA6120 works fine in single-ended design. I have built such an amplifier and it sounds pretty good for its small size, but it can't compete with my discrete amp. I have a thread on Head-Fi documenting it if you want to see my implementation.

Be careful of the large input bias current of this opamp. It is not documented in the datasheet, but is large enough to cause pretty severe DC offset on the outputs. If you're designing around the chip, it would be wise to include a DC servo or be very careful balancing input currents.

I do still have (poorly manufactured - arrgh) a couple boards available if you're interested, but I suggest if you choose to use one that you ignore my parts values and calculate your own since I have modified the amp considerably since doing up the schematic, and it has offset issues.
 

jcx

Member
2003-02-17 7:38 pm
..
rather than DC servo - the TPA makes a fine output amp in a multiloop amp, Jung actually uses cfa op amps with V gain in most of his multiloop designs - rather than the unity gain buffers that the Headwize crowd has fixiated on/obsessed about - and the TPA can be used unity gain too
 

jcx

Member
2003-02-17 7:38 pm
..
Dxvideo said:
... However, I haven’t seen any "unity gain" current feedback amplifier... Are you sure that?

Yes current feedback amplifiers can be used unity gain - but you need to keep a Rf resistor in the circuit - not just short -in and out like a voltage feedback op amp

in a cfa the feedback resistance seen at -in determines the dominant pole of the loop, that is why they can be so fast "optimum decompensation" is built into the circuit by the reduction in feedback Z as the gain is increased with constant Rf

by using larger value Rf resistor cfa op amps can be "overcompensated" and work fine at unity gain

see the TPA6120 data sheet fig 27

from the identical chip THS6012 data sheet:

"As with all current feedback amplifiers, the bandwidth of the THS6012 is an inversely proportional function of the
value of the feedback resistor. This can be seen from Figure 17 through Figure 20. The recommended resistors
with a ±15 V power supply for the optimum frequency response with a 25 Ω load system are 680 Ω for a gain = 1
and 620 Ω for a gain = 2 or -1. Additionally, using a ±5 V power supply, it is recommended that a 1-k Ω feedback
resistor be used for a gain of 1 and a 820 Ω feedback resistor be used for a gain of 2 or -1. These should be
used as a starting point and once optimum values are found, 1% tolerance resistors should be used to maintain
frequency response characteristics."

This isn't just theory, the TPA6120 is excellent in unity gain application because of its large input common mode range which allowed me to use 1/2 of the TPA op amp in my Class A amp output stage in unity gain:

http://www.head-fi.org/forums/f6/ad8397-class-188758/index2.html#post2263321

and a few more jcx posts in that thread; one (#16) shows the voltage feedback version of the Class A bias circuit I built with the TPA and 1.5K Rf instead of the feedback short circuit shown in the sim's U2 Lt1128 vfa unity gain follower circuit

slight over compensation improves overshoot/stability into C loads but these op amps need a load isolating impedance between their output and any significant C - see the 10 Ω series Ro in the TPA eval circuit- I used a lossy bead core EMI suppressor/ inductor to get near zero output Z at audio frequency