simple opamp circuit help - Page 1
Forum section: electronics / beginners

author: dfox416
author_rank: Contributor
timestamp: November 11, 2013, 08:47:24 am
content: I am designing an audio circuit design and it starts with an opamp at the input. (I attached a schematic for the opamp configuration I tried) I built an inverting opamp circuit on a breadboard and measured the output with an oscilloscope and with a speaker but got nothing. Sometimes the opamp will suddenly start to work, giving the expected gain, but if I turn the power off and back on, it won't work. It seems like it might have to do with the capacitors charging up but I am not sure. As an input, I am using a 1kHz sine wave from a signal generator with a 600 ohm output and I am supplying 9V to the opamp. Please help, I really don't understand what is going on!

author: tszaboo
author_rank: Super Contributor
timestamp: November 11, 2013, 09:00:04 am
content: You dont have a stable DC operating point. Lower the resistance of R1-R2 by a magnitude, and connect C1 with a high resistance to ground or half rail.

author: dfox416
author_rank: Contributor
timestamp: November 11, 2013, 09:17:02 am
content: Thank you so much. It seems to work reliably now. However, it takes about 10 seconds to start working do you know why? Also, would the high resistance to ground/half rail come before or after the capacitor?

author: Jebnor
author_rank: Regular Contributor
timestamp: November 11, 2013, 02:34:43 pm
content: Quote from: dfox416 on November 11, 2013, 09:17:02 amAfter the first capacitor.

author: drtaylor
author_rank: Regular Contributor
timestamp: November 11, 2013, 03:27:29 pm
content: It takes a long time to start because you have a huge time constant with your capacitor values. The only source to charge the capacitor to its eventual dc point is through R2+R1. 1 time constant is 242 seconds. So you experience a long wait before the stable point has been reached. For audio there is no need for such a large coupling capacitor. I'd recommend around 1uF which would make a pole < 20Hz. The value of C2 should be calculated with the expected load resistance. The value of the resistors is greatly dependent on the type of Op Amp. But I don't agree with the previous post necessarily. Bias current considerations might be important depending on the Op Amp type. To achieve good low noise performance, R1 and R2 are too high and will generate noise from just the resistors. So for further help, publish the Op Amp type and also don't forget the bypass capacitors.

author: tszaboo
author_rank: Super Contributor
timestamp: November 11, 2013, 06:09:33 pm
content: Quote from: drtaylor on November 11, 2013, 03:27:29 pmI've just seen it it is 220uF, that is indeed too much. A circuit should not rely on the bias currents for operating point, for a lot of rasons. The opamp is TL072 as far as I see.Yes, the resistor(s) go after the first capacitor.

author: drtaylor
author_rank: Regular Contributor
timestamp: November 11, 2013, 07:31:21 pm
content: The TL072 has negligible bias current...<7nA full temp range. There is no need to modify this circuit except for the size of the Caps. The bias current is easily supplied by R2 but up to 70mV of offset can happen which will vary with temperature. This is reduced to <7mV if R2 is 100k instead. Due to all AC coupling this is probably acceptable. If R1 and R2 are reduced an order of magnitude to 10k and 100k then the input cap should be 1-2uF for full range bass. There is no need for an additional resistor. An additional resistor to the 1/2 ground point will increase noise gain needlessly. I'd add a 0.1uF cap across R4 to reduce supply noise at pin 3.

author: chasxmd
author_rank: Contributor
timestamp: November 12, 2013, 12:56:56 am
content: I love Op Amps.. you'll really be doing yourself a favor if you watch w2aew's video:and perhaps keep a copy AN-31http://www.ti.com/ww/en/bobpease/assets/AN-31.pdfaround for reference...Of course those are free resources.. I have a great circa 80's book that is better but I imagine you know how to use Amazon if interested in a book.