2
\$\begingroup\$

I'm simulating an audio amplifier with uA741 in pspice (student version). The input signal is a 0.1v (100mv) sine wave with frequency between 1kHz and 10kHz. The theoretical gain is 33 but in simulation, the gain of output is only 3 and we have DC shift and distortion. The slew rate is 0.5 v/us and if we calculate the max slew rate needed for my input signal using the formula below, we get a value much smaller than slew rate of uA741.

slew rate = 2 PI x f x Vpk /10^6 = 0.006283 v/us http://en.wikipedia.org/wiki/Slew_rate

So the problem is not slew rate. When I replace the uA741 with "OPAMP" in pspice (which is ideal opamp) there is no distortion in the output. What do you think is the reason for this distortion?

Here are the schematic and input(red)/outputs(green) graphs: enter image description here enter image description here enter image description here

\$\endgroup\$
  • \$\begingroup\$ just increase the load resistor to 1k or reduce the input voltage \$\endgroup\$ – yogece Oct 21 '13 at 5:26
  • \$\begingroup\$ The 741 (like nearly all op-amps) is not a power amplifier and is not intended to drive a low impedance speaker load. \$\endgroup\$ – Andy aka Oct 21 '13 at 7:14
5
\$\begingroup\$

You are seriously overloading the 741. The datasheet says the typical output short-circuit current is only 25 mA - with your 8 ohm load, that should only give a 400 mV peak-peak output - and that is assuming a +/-15 volt power supply.

\$\endgroup\$
0
\$\begingroup\$

I know, it's a old thread, but maybe usable for other visitors.

Your answer is already here. Your input have a too high level. You should to come in input with a lower level, or reduce the gain of the apamp by drastic decreasing ratio of the feedback resistors loop (in fact, you should to reduce R5 resistor). (second option is better for the noise level).

But in facts, UA741 isn't a good opamp for audio and it is completely obsolete, noisy, slow... My opinion: this circuit should have already been forgotten! Many opamps with better caracteristics exists now, in the same price range. TLxxx series for example. And if you agree to spend $ 1, you can get a NE5532, whose performances ridicule those of UA741 in this application.

But...

In your case, the LM386 is especially designed for that and needs very few external components to work. Don't forget to read the datasheet.

If you want a very high impedance input, multiples solutions exists: - Place a TL071 as follower assembly between the source and the input of LM386. - try a TL071 with a additionnal stage output AB-class amplifier. A very classic configuration; only 4 transistors and 5 resistors. Something in this style: https://sonelec-musique.com/images/electronique_ampli_casque_003.gif - Many many others solutions... Be imaginative !

But seriously, reship the UA741 at the 70's...

Sorry for my acrobatic english.

\$\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.