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Elvee 30th August 2012 02:14 PM

A small, low-voltage, battery operated amplifier
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Here is a small, low-voltage (3V or less) amplifier.

I wanted a signal-tracer amplifier, very small, and battery operated to be used in completely floating situations: like typically to measure the noise across a component.

This amplifier fits the bill. It is rather crude, but has some interesting features: it uses same-sex output transistors, operates efficiently at ridiculously low voltage levels, and yet offers a decent quality: here, it is shown at max output power, and the THD is well under 2%, with a nice harmonic profile.
At lower power, the distortion is even lower.
Also note that the gain is very high, because of the intended application.
More reasonable values would further decrease the THD, and increase the input impedance.
The peak to peak output voltage before clipping reaches 2.5V with a 3V supply, which is quite impressive

No temperature compensation is shown; for optimum stability, R4 should be an NTC (or an NTC should be part of R4).
But even without compensation, the stability is acceptable thanks to the low supply voltage: the total quiescent current establishes itself at ~10mA.

The polarity of all transistors could be reversed, allowing the use of germanium transistors: the only adaptation required is R4, that should be changed to 39 or 47 ohm.

R9 is optional, it slightly improves the THD and the stability.

Have fun!

rjm 31st August 2012 06:39 AM

R1 appears to load the output with 4 ohms. Is this correct?

Elvee 31st August 2012 12:54 PM

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Originally Posted by rjm (
R1 appears to load the output with 4 ohms. Is this correct?

R1 symbolizes the load (speaker). The circuit has been designed for a minimum load of 4 ohm, but in practice it will accept even lower loads.
For an 8 ohm load, BC337-25 are sufficient.
If the amplifier is used exclusively with 32 ohm earphones, all the transistors can become BC547 or similar.

This original version is rather sensitive to the supply voltage.

The following variant eliminates this sensitivity completely at the expense of an additional transistor.
It is highly unorthodox, but it has some side benefits: it also compensates thermally the output stage, and provides a further reduction in distortion.
R4 has to be adjusted individually to set the quiescent current, much like the Vbe multiplier of a "normal" amplifier.

rjm 31st August 2012 04:52 PM

I see now. I wasn't sure earlier because a) you indicated this was for instrumentation, so I could see no reason for driving 4 ohms and b) it's tied to the positive rail.

Point b) above rather makes is unsuitable for headphones, unfortunately, at least conventionally wired ones with shared return wire.

Perhaps your circuit might find some application as the driving amplifier for little self-powered computer desktop speakers?

Elvee 31st August 2012 08:01 PM


Originally Posted by rjm (
Point b) above rather makes is unsuitable for headphones, unfortunately, at least conventionally wired ones with shared return wire.

It does not matter: your headphones don't care about the absolute potential (if such a thing even exists) of the common return wire.
Anyway, the whole thing is supposed to be battery operated = floating.

The only case it could pose a problem is if the return of your headphones is hardwired to something tied to the input ground.

rjm 1st September 2012 11:27 AM

Thinking about it some more, you are correct. Just have to avoid the jacks which connect the return to the case...

Elvee 5th September 2012 05:29 PM

2 Attachment(s)
Here is the final incarnation of this amplifier in the signal-tracer.

I have used the transistor-compensated version, added a FET buffer, volume pot, input protection, 3 NiCad and their rudimentary charger.

The whole thing is built into the shell of an old, eighties-style touch-phone.

Works like a charm, you just have to unhook to power it, and there is also a possibility to connect earphones or an external speaker.
Quiescent current consumption is 9mA.

The quality is not bad for such a rudimentary circuit: I tried it on the big lab speaker and it made quite an impression. On 8 ohm, it reaches 2.7Vpp before clipping (with a 3V supply)

danielwritesbac 6th September 2012 07:53 PM

Can I use it with a 5v supply?

Elvee 7th September 2012 07:34 AM

1 Attachment(s)

Originally Posted by danielwritesbac (
Can I use it with a 5v supply?

Yes you can: the output DC level has to be modified to 2.5V with R6, and R4 (quiescent current) needs to be readjusted.

The value on the schematic is only indicative, you have to adjust it individually. The adjustment is quite sensitive, but once it is done it remains perfectly stable.

If a fixed 5V supply is used, then the resistor/NTC compensation is sufficient

danielwritesbac 7th September 2012 12:14 PM

Is MC7805, or maybe MC7806 okay to use?
I forgot to ask about max operating voltage when driving 8 ohm speaker.
Is that amp an NTP input or a singleton input?
And, would it be okay to bridge it or is the THD a bit too high for that?
Other problem: It hangs up when it touches my face. :)
Figured a name for it: Tracerphone! :)

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