variable amplifier
variable amplifier
(OP)
I'm currently trying to put together a variable gain for volume control of a medical listening device (20-2kHz). The preamp is a low voltage op amp LM6582 wired in an inverting configuration -(Rf/Ri). The adiuo amplifier is a low voltage LM4860 1W essentialy an inverting configuration. The problem occurs when placing a pot in the feedback path. The signal is attenuated nicely until about 35% of the maximum value of the pot. The signal at 35% goes to the rail 3v. The feedback path will handle a nominal valued resistor that is equal to the maximum value of the pot, so the device is not near its maximum gain. These devices have not been wired together and remain seperatley tested until I get a nice volume control out of on or the other. I am looking to use these two devices for this solution. I may have to use a voltage divider of the electret microphone signal. Any thoughts sent my way would be helpful.





RE: variable amplifier
now you said you're using a pot in the feedback loop of the amp? instead use a pot as Ri, this is a method that i have done before with a 741 and it worked fine all the way up to the rail voltage. if you have a dedicated Rf and Ri and then a pot in the feedback, then you are effectively attenuating the feedback, right? well you can only go so far with that before the amp doesn't see any feedback and BAM , slam the rails. if thats how you've got it set up then this may help...otherwise i've wasted your time.
RE: variable amplifier
Thanks for the reply I will place the pot in with the input resistor and give that a try. I still am not sure about the output going to the rail at about 35% of the pot's nominal value. The pot's nom value is 100k and truns down to zero. If the pot is the only device in the feedback path what causes the railing at about 35%. Theory: -(Rpot/Ri) if Rpot is zero Ohms voltage follower. If Rpot is maximum 100k and Ri is 10k we see a gain of 10 180deg out of phase.
RE: variable amplifier
okay, i think i misunderstood exactly how the circuit was setup initially. from what i gather now your Ri is 10k and your Rf is the pot. this makes sense. so the amp hits the rails at 35% of the pots max value...so 35k, at this point the amp is set up for a gain of 3.5. two questions,
1) What are your supply voltages for the amp
2) How large is the input signal(max)?
is it possible that at a gain of 3.5 you are already within 2V of the supply? maybe your problem lies within the supply voltage rather than the resistor network. from what i gather you have a supply of roughly 5V correct? so if you are hitting the rails at a gain of 3.5 your input is somewhere near 860mV RMS. if this is correct then your problem lies at the supply voltage, if this is not correct then i would suspect your problem to be with the IC and not the circuit.
RE: variable amplifier
RE: variable amplifier
RE: variable amplifier
Also, the capacitance of the feedback loop will change with the position of the wiper, becoming less with more ohms and becoming greater with less ohms.... just what you want to occur to keep the noise BW within reasonable limits.
RE: variable amplifier