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an opamp question

Started by whitty, July 01, 2009, 07:21:52 AM

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whitty

hi team. i have a tl702 dual opamp, on the datasheet in the vcc field it says +/- 18 volts. does this mean positive and negative 9v supplies(18v peak to peak)? if thats the case how do i use negative battery terminal for ground AND neg 9V?? confused. thanks in advance.

petemoore

  I would leave some room for 'surge' and margin of error.
  Stack two 9v batteries by connecting a - from one, a + from the other, call it 'ground'.
  That leaves 2 of the 4 battery poles you started with, open.
  One is 9v 'above' the ground.
  The other is 9v 'below' ground.
  9V+
  Ground
  9v-
  that way when the signal wants to swing - the active has between Ground and 9v- to swing it [less whatever headroom the amp requires].
  Same thing when swinging the output positive, having room between Gnd. and 9v+ to do work.
Convention creates following, following creates convention.

R.G.

Quote from: whitty on July 01, 2009, 07:21:52 AM
hi team. i have a tl702 dual opamp, on the datasheet in the vcc field it says +/- 18 volts. does this mean positive and negative 9v supplies(18v peak to peak)? if thats the case how do i use negative battery terminal for ground AND neg 9V?? confused. thanks in advance.

First, I believe that the section of the datasheet you're reading is labeled "Absolute Maximums" or something similar. What that means is (a) a bipolar power supply of both 18V above ground applied to the V+ terminal and -18V (below ground) applied to the V- terminal; this is a total of 36V, and they're telling you that if the combined power supply difference between V+ pin and V- pin is over 36V, it is likely to damage or destroy the chip. This is not intended to be the only power supply voltage you use, only a "don't exceed this" number. Like a road sign that ways "Speed limit 65". You can exceed it, but undesirable things may happen if you do.

The TL072, like many opamps is designed for use with bipolar power supplies, meaning balanced above and below ground. However, the opamp does not know where "ground" is; it only knows where its power supply pins are and where its bias voltage is. The bias voltage is the voltage applied to the noninverting (+) input. If this voltage is exactly in the middle between the power supply pins, the opamp cannot tell the difference. So you can power it from 0V and 36V if you supply it 18V on the (+) pin.

And you can make it work from smaller power supply voltages. There is probably a minimum power supply voltlage specified on your datasheet if you look for it. This is the lowest voltage between the V+ pin and the V- pin where everthing inside works as expected. For the TL072, 9Vdc from a battery on the V+ and V- pins and a bias voltage in the middle of 4.5V works just fine. In that case, the V- pin can be 0Vdc, and if you use a DC blocking capacitor between 0V/Ground and the + input which is biased at 4.5V, the opamp will work just fine. This is in fact how most opamps in pedals work.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

whitty

so the -vcc pin goes to earth, and the +vcc pin goes to 9v, this creates the 4.5v bias. and the cap has positive leg on +vcc pin and negative leg on -vcc pin. correct? it dosnt seem quite right.. sorry to make you explain it again.

R.G.

Almost. The +Vcc pin (pin 8) goes to +9V. The -Vcc pin (pin 4) goes to the negative terminal of the battery, which is connected to the signal ground from the input and output jacks. You must create the +4.5V bias voltage yourself. This is usually done by using two equal value resistors in series from +9V to ground. The center connection of the two resistors sits at +4.5V. This is usually bypassed to ground with a largish capacitor, 10uF and up. With this bias voltage created, you can run a bias resistor to the (+) input of the opamp.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.