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Old 7th December 2003, 11:53 AM   #1
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Default OPA2604 on Single Supply...??

I've searched and read, and still can't find a definitive answer. I have an old receiver that uses a pair of BA312 opamps in the tone amplifier, one is dead. The supply is +35V. Supporting circuitry will have to be modified a bit, but I want to replace both opamps with a single OPA2604.

Am I going to have to build a zener +/- supply, or will the 2604 behave in a single supply environment?
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Old 7th December 2003, 01:16 PM   #2
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Every opamp can be used in single supply situations as long as the common mode range of the inputs not are exceeded. => you must bias the inputs in some way.
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Old 8th December 2003, 01:10 AM   #3
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This is what I wanted to do...nothing special, but generally split supplies are used.

So what's the problem with this?
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Old 8th December 2003, 11:36 AM   #4
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Bump.

Anyone? Would this function properly?
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Old 8th December 2003, 12:17 PM   #5
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This is not OK. You must insert a cap in the feedback down to ground and you must pull up the input at half the supply voltage.

What happens if you have negative input voltages? Think about that.
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Old 8th December 2003, 09:02 PM   #6
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Something like this? How about feedback cap size?
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Old 8th December 2003, 10:45 PM   #7
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OK, I can see that with the low values of feedback resistors that low frequency rolloff with be too extreme unless cap size gets fairly large. Increase the resistors to the value here, and bump the cap size up a bit.

Sorry for what seems like stupid questions, but I honestly didn't know you could do this. If this works, it will save a lot of trouble.
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Old 9th December 2003, 05:27 AM   #8
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The above circuit works peachy. Your assistance is very much appreciated peranders.
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Old 9th December 2003, 07:41 AM   #9
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Your input works only if your PS is quite, otherwise you will get noise and hum right in.

Better solution is:

2k2 + 2k2 creating half the voltage. From the junction down to ground 100 uF or so. From the junction 100 k to the + input

The caps is calculated f = 1/(2*pi*R*C) => 10 uF, 1 K => 16 Hz
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Old 9th December 2003, 08:33 AM   #10
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Quote:
Originally posted by peranders
Your input works only if your PS is quite, otherwise you will get noise and hum right in.

Better solution is:

2k2 + 2k2 creating half the voltage. From the junction down to ground 100 uF or so. From the junction 100 k to the + input.
This is run off of a regulated supply that is well bypassed and is actually quite noise-free. Additionally, the circuit shown above lends itself quite well to the current component layout. Your suggestion would be excellent if there was room on the board, but component layout would become pretty cumbersome. I may try it next time if I can find the board space.
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The caps is calculated f = 1/(2*pi*R*C) => 10 uF, 1 K => 16 Hz
Which is why the resistors were upped to the values shown in the last schematic, and the cap increased to 47µf.

As it runs now, using 100MHz bandwidth on the scope, opamp output noise reads about 15mV P-P. Cut bandwidth to 20MHz, and noise reads about half of that. Sound quality is excellent, so I'm leaving good enough alone for now.

Thanks again for your help and comments. Being able to single-supply this opamp has saved me a lot of trouble.
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