0
\$\begingroup\$

I'm trying to choose a good op amp for a Class A small signal audio amplifier circuit.

I've done my research and understand the use pull up or pull down resistors externally to force Class A operation, and other specs influencing op amp choice generally.

What I can't find in my research is this - when choosing an op amp for use in a Class A design, is there an advantage to one type of internal op amp output stage type?

e.g.,

  • rail-to-rail emitter follower
  • totem pole (NPN-NPN)
  • common collector (NPN-PNP)

enter image description here

\$\endgroup\$
  • 3
    \$\begingroup\$ For me the last sentence makes this smell a little too much like a homework question as you can find the answer (to that last sentence's question) in any good book about amplifier design. We will not "do" your homework for you here so you will have to find the answers yourself. If you have a more detailed question you cannot find an answer to, you are welcome to discuss it here. \$\endgroup\$ – Bimpelrekkie Mar 11 '18 at 20:13
  • 2
    \$\begingroup\$ It's not clear whether you are asking about the design of the opamp, or the circuit around it. \$\endgroup\$ – Olin Lathrop Mar 11 '18 at 20:26
  • 1
    \$\begingroup\$ There may be current feedback or limiting , it depends, usually it defeats the purpose of ultralow standby current , but sure, that's a definite maybe or maybe it will become unity gain unstable. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 11 '18 at 20:45
  • \$\begingroup\$ I'll address the questions & do an edit. It's not homework, I'm not a student, just a newbie here. \$\endgroup\$ – Samuel Barns Mar 11 '18 at 22:35
1
\$\begingroup\$

Converting a class B output stage to class A is pretty straightforward - just add a low-value resistor to one supply or the other. For instance

schematic

simulate this circuit – Schematic created using CircuitLab

causes the internal drive level to stay positive for any output level greater than the Thevenin Equivalent voltage of the load and bypass.

It should also be pretty clear that this will dissipate considerably more power than the standard output, but that tradeoff is well-known.

It should also be noted that this may run into op amp output stage current limits. For large positive voltages, the NPN must provide much more current in the modified version, and this may not actually be possible for any particular op amp/load/bypass configuration.

\$\endgroup\$
  • \$\begingroup\$ The output current limitations can be avoided by using a current sink in place of the bypass resistor. \$\endgroup\$ – Jonathan S. Mar 11 '18 at 23:35
  • \$\begingroup\$ This works well, as I use a 10 K pull-up for bipolar op-amps running on a single supply. It creates an offset current that removes severe distortion. \$\endgroup\$ – Sparky256 Mar 11 '18 at 23:55
  • \$\begingroup\$ Note that the question was not "How do I.." I will edit it again to try to make this clearer. \$\endgroup\$ – Samuel Barns Mar 12 '18 at 14:55
0
\$\begingroup\$

Another, related, topic on opamp distortion regards the input diffpair linear range. At very high frequencies, substantial Vinpp (between Vin+ and Vin- pins) must appear, because the opamp gain has become quite low at frequencies approaching Unity Gain Band Width.

Such legacy high-performers as the UA715, which is spec'd to provide excellent settling performance, depends on a wide input range so the opamp circuit can switch from slewing to a clean settling behavior.

This same wide input range reduces distortion at high frequencies.

Thus if you ignore the input voltage linear range, the act of biasing the output devices into classA will not provide the clarity of music etc you were expecting.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.