OPAMP Amplification

[Dylfish]

Hey guys, expermenting with circuts so we thought just for learning in baby steps we would do this.

anyways i went out to jaycar and got a tl071 opamp and an assortment of diodes (1n914, and a few germaniums) and anyways, connected the opamp up, then we wanted to test if it amplified correctly before we added the clipping section afterwards. (the only difference was the resistor values used ( 82k and 11k ))

anyways, we wired it up (bread board) and all we got was radio interference (made my amp a great AM radio :S) and ALOT of noise and buzzing with no signal comming through. We have tried with a few opamps so i know it not fried. Im sure its how we got it set up. i was wondering if anybody could help us with the pinout and what could be causing the RFI?

Plus do i need a DC filter on the input and output for it to work?

[sfx1999]

You have the + and - reversed. Another thing to note is that the ground is not actually the negative terminal of your battery. You need to either use a voltage divider to get 4.5 volts, or to hook two 9 volt batteries in series and connect the ground to where the batteries are connected.

[EDIT]

Actually, you can't use a voltage divider unless you want to use an inverting amplifier. The voltage divider will be thrown off by the voltage coming from the feedback.

[dschwartz]

yup, i see the same errors,..

1.- the feedback goes to the - input
2.- the signal goes to the + input
3.- Connect a 4.5 volt (or half supply voltage) source to + input
4.- Put an output cap (~1uF) to avoid any DC bias.
5.- put a small value cap (~150pf) between the + input and ground to avoid radio freq getting in..
6.- put a cap in series between the input signal and + input to eliminate any dc coming in or out..22n will do fine

[R.G.]

TL071 pinout:

1 = N/C (no connection)
2 = inverting (-) input
3 = noninverting (+) input
4 = most negative supply voltage

5 = N/C
6 = output
7 = most positive supply voltage
8 = N/C

Considerations:
1. This and most other opamps are designed for bipolar ("bi" = two, "polar" = sides, ends, faces, etc; "bipolar" = two power supplies symmetrical around some reference point we call ground); in the example circuit, this can most easily be done by using two 9V batteries which are connected with one + and one - terminal together. The connected batteries are wired to the ground symbol in your circuit. The free + end becomes the most positive power supply, the free - end becomes the most negative power supply.
2. The + and - inputs must be biased near the middle of the most positive and most negative power supply voltage. In the circuit you show, this can be done by connecting a large value (perhaps 1M) resistor from the + input to ground. The - input is already referenced to ground by the resistor drawn from it to ground. Notice also that as shown your circuit has a gain of 7 to both AC and DC signals. So any DC on the + input is multiplied as well. It would make a great deal of sense to put a capcitor in series with the 10K resistor from the - input to ground. This capacitor will block any DC gain and force the DC gain to be one. The - input is then biased properly by the output of the amplifier through the feedback resistor.
3. Your input signal should either not have any DC component on it (all guitars do this naturally) or should have a capacitor in series to block any DC. Any DC on the input will shift the bias you have set up with the 1M resistor to ground and may foul up the results.
4. Your gain may not be high enough to make the diodes distort. Guitars have a maximum output signal of perhaps 100mV PEAK for single coil pickups. A gain of seven puts that at 0.7V, which may or may not cause noticeable clipping with silicon diodes as you show. I suggest you change your 10K resistor on the - input for a 10K pot. Changing the pot setting will let you experiment with different values of gain. Use the capacitor I referred to in 3 with the pot as well. Make it biggish, maybe 22 to 100uF.

I think your RF problems and lack of signal are because of the power supply and biasing addenda that are not fully laid out in the circuit.

If you must use a single 9V battery, construct a synthetic "ground" by putting two 10K resistors in series across the battery. The + end of the battery is then the most positive power supply, the - end is the most negative, and the center of the two 10K's is "ground". Connect a largish (10uF to 100uF) capacitor from the "ground" to either end of the battery, observing polarity.