Considerations when moving from single to multiple stage amplifiers

Narwash

Joined Jun 27, 2012
32
Hi all,

I'm building a simple two stage, +5V single supply, amplifier circuit. It consists of two operational amplifiers in the non-inverting configuration. It is powered by a 9V battery going to a 5V regulator. I was just wondering if I need a current amplifier stage in order to produce the necessary current to run a two stage amplifier versus a single stage. I suppose my question is: do I need twice the current to run a two stage amplifier as opposed to a single stage? They both operate at the same voltage so I figure that, since they're in parallel with identical components, they, in theory, need double the current that a single stage amplifier would. So I figure I could have a simple common emitter BJT amplifier stage with a beta coefficient of 2 inbetween the 9V battery and the 5V regulator. I figure the drop in voltage across this amplifier will still be in the operating range of the 5V regulator.

So that's my game plan so far. I was wondering if anyone had any insights into flaws in this plan or potential pitfalls. I've been pretty careful about decoupling and power supply bypassing.

panic mode

Joined Oct 10, 2011
2,530
twice the load, twice the current. what opamps you have there?

Audioguru

Joined Dec 20, 2007
11,248
A transistor will not "amplify" the current from the battery and make a weak old battery produce lots of current like a new battery.

If the battery does not produce enough current then replace it with a new strong battery.

With a 9V battery and a 5V regulator, use a low-dropout regulator.
An ordinary 7805 regulator has a minimum input of 7V when the battery still has lots of life remaining.
A low dropout regulator has a minimum input of 5.5V or a little less so the 9V battery will last much longer.

Narwash

Joined Jun 27, 2012
32
Awesome. Using a low dropout regulator is a great idea. I believe I understand the problem now. When I'm doubling my load, the circuit will draw twice the current, as compared to a single load, and the battery will last half as long. Makes sense I guess I just over-thought it. Thanks for all your help!

atferrari

Joined Jan 6, 2004
4,731
Two stages or two opamps in parallel?

What if you show a circuit?

MrChips

Joined Oct 2, 2009
29,875
The 5V regulator is wasting energy. I would skip it.

Narwash

Joined Jun 27, 2012
32
@atferrari: There is two op-amps operating at the same voltage, but the output of the first is wired to the non-inverting input of the second. I believe I am cascading the two and approximately doubling the gain. They have the same feedback configuration. The two stages are decoupled through a pair of .1 uF and 2.2 uF capacitors in parallel between the output of the first op-amp and the input of the second.

@MrChips I don't know an alternative to a 5 V regulator. How else would I make sure my op-amps are getting a steady 5V as the battery voltage drops? I figure the battery drops from about 9.6V at full charge to around 6V when it's on it's way out.

MrChips

Joined Oct 2, 2009
29,875
The gain of a properly designed amplifier is independent of supply voltage. You do not need to regulate the supply voltage depending on your application.

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