I'm trying to build a very simple circuit, that will amplify an audio signal coming from a 3.5mm jack and play it back on a small 32ohm speaker.
I can see that the amplifier is actually amplifying, if I compare the strength of the input voltage versus the output voltage. But the problem is, it's amplification level appears to be only a factor of 6x. Based on the R2 / R1 division, the amplification factor should be around 100x, correct? if I crank up the voltage to 15v, then the amplification factor increases, but not significantly. Why is the amplification factor so low, even through the datasheet states the the amplification factor is calculated based on: R2/R1?
Another problem is that the speakers output is as quiet as an output from headphones. Even if I crank up the power supply of LM318 to 20v – the speakers volume doesn't increase.
As far as I understood, I will get very limited current supply out from the op-amp and won't be able to drive the speaker directly, correct? Do I need to drive the speaker with a transistor to make it louder or what would be the proper method? Thank you for you time.
Best Answer
The LM318 is not designed to drive the speaker directly .You could make a NPN / PNP follower that has a voltage gain of almost unity .The proposed follower would have a high current gain .The actual current gain is roughly the HFE of your transistors .This would raise the impedance that the LM318 opamp sees to several thousand ohms depending on transistor gain.Now the opamp will perform as predicted and your gain equation will be valid .If you biased the follower well into class AB you should have something that will sound better than a chip at the expense of increased power drain .