I built an audio amplifier based on TI's TAS5630 IC. This is a Class-D amp with multiple configurations. I'm using it as a 2-channel amp. The problem is that this configuration requires a differential input. So I built this little circuit that seems to do the job: circuit

The outputs are between C2/R6 and C3/R7.

Is it worth designing an opamp based circuit or is this "good enough"?

  • \$\begingroup\$ 2N2222 is just used as an example. I'm open to suggestions as well. I'd rather build this as a little SMD board with BC847. I only have "common" transistors such as these and BC547/8/9. \$\endgroup\$ – hjf Apr 10 '17 at 22:44
  • \$\begingroup\$ You would use a Complementary BJT, or IGBT or MOSFET full bridge. What impedance for 20Hz and 30kHz? HOw many Amps? What efficiency? How much EMI? What bandwidth? What LINEARITY? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 10 '17 at 22:53
  • \$\begingroup\$ Never let Vce<2V for good linearity due to drop in hFE. partial saturating effects rise from Vbe to several volts with Ic due to effective rCE of BJT's \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 10 '17 at 23:01
  • \$\begingroup\$ For linearity, reduce the base voltage from VDD/2 to VDD/3. This ensures approximately Vdd/3 across the transistor Ve, as Tony Stewart advised. For bias-point stability as Beta changes, reduce those 2 resistors R3 and R4, to be on order of R1 * 10, instead of R1 * 100. Thus try R3 = 10K; R4 = 5.1K. \$\endgroup\$ – analogsystemsrf Apr 11 '17 at 5:15
  • \$\begingroup\$ Sorry for commenting an old question but could anybody explain the purpose of R5 in the above circuit? I have seen a 100k resistor to GND also in similar circuit - does it not add unnecessary noise to the signal? \$\endgroup\$ – Junius Feb 24 '19 at 13:18

I was puzzled at why your simulation should work as well as it appears to, given that R3 and R4 are identical, setting the base to more or less mid-rail, which means you have no voltage across the transistor. However, if it happens, it must be true.

Then I realised that the transistor is probably drawing enough base current to move the base bias point down just enough to work with those small signals. However, this gives you little headroom, significant distortion on the collector due to the low Vce, and you can't guarrantee you wouldn't get a transistor with much higher beta (finally a circuit where too much beta is bad!)

The solution is to explicitly move the base voltage down, with perhaps R3/4 as 100k/47k to get nominally 1/3rd of the rail. This gives you a decent Vce and signal swing, and for any practical beta.

I would always be happy with a circuit like this, even though the distortion would still be measurable. It's insignificant compared to the distortion a loudspeaker can generate, and it's tame, very low order.

With this large supply voltage, you could get quite a pop as you power on, unless you take steps to mute the power amplifier initially. Make C2 and C3 as small as possible consistent with transmitting your lowest bass notes to minimise the time it takes to charge them. At the moment, if we assume the R6 represents the load, the time constant is 100mS, so frequency response going down to -3dB at 1.6Hz, which is overkill.

A balanced rail circuit could be made to deliver less pop on power on, and an opamp could deliver less distortion, but unless you use a decent (expensive) one, noise will be larger (but probably still insignificant compared to the power amplifier).


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