Electronic Circuits and electronic circuits, electronic schematics plus an extensive resource for hobbyists, inventors and engineers

DiscoverCircuits.com, has 45,000+ electronic circuits, cross-referenced
into 500+ categories.    We have searched the web to help you find quick design ideas.
We make every effort to link to original material posted by the designer. 
Please let us if you would like us to link to or post your design.

HOME Schematics Index Hobby Corner Dave's Circuits Electronic Resources Contact Info
Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches

Ultra-low quiescent current AF power amplifier

All materials and content contained on this page are the exclusive intellectual property of Andrew R. Morris and
may not be copied, reproduced, distributed or displayed without his express written permission.

This circuit is best used in a non-high fidelity application (i.e. speech audio) where battery life is critical. This is an audio power amplifier, similar to what is common in transistor radios and other portable audio equipment. This circuit however, has an ultra-low quiescent current of about 300uA, due to a unique bias circuit. I don’t even know of a chip that does that. This circuit also has much higher open-loop gain, due to the bootstrap effect that the circuit provides as a by-product of its operation.

Diodes D1 and D2 perform the usual task of closing the gap between the point where Q7 or Q8 stops conducting and where the other transistor starts conducting. This gap causes an effect known as “crossover distortion”. To prevent crossover distortion, Q5 and Q6 would normally be slightly in simultaneous conduction when no signal is present, wasting precious battery power. Because precise control of the quiescent current is extremely difficult, it is always adjusted such that significant power is wasted. This circuit uses an extremely tight control loop to maintain the proper voltage across D1 and D2 virtually eliminating crossover distortion with almost no simultaneous conduction of Q7 and Q8. I say “virtually”, because under heavy load, a tiny amount of crossover distortion can just be seen on the scope, but would be completely inaudible.

Q3 and Q5 form a regenerative (infinite gain) amplifier, similar to an SCR. It would indeed latch up like an SCR without negative feedback from Q4, which senses the voltage across D2. D1, having essentially the same current, will have essentially the same voltage across it. R5 provides the start-up current for Q3, which causes Q5 to conduct. This in turn, causes Q3 to conduct more, causing Q5 to conduct more, producing the previously mentioned regenerative effect.

The voltage across D2 will keep Q4 just barely in conduction. Since the diodes and transistors are all silicon, the voltage across D1 and D2 will also be just exactly the voltage necessary to keep Q6 and Q8 just barely in conduction. No wasted power. Note that D1, D2, Q4, Q6 and Q8 must be in the same temperature environment. For larger amplifiers, they should be on the same heat sink. In really big amplifiers, D1 and D2 should be replaced with 1N400x types. The quiescent current will be proportionately larger.

If you want more quiescent current, or need emitter current limiting resistors, you will have to add a resistor in series with D1 to get the added voltage. The drop across the resistor will be constant because the diode current will be constant. Due to the infinite gain of the Q2 and Q4 combination, quiescent current is essentially unaffected by battery voltage.  Note that the circuit works poorly if the diodes are replaced with resistors. The circuit works because the non-linearity of the diodes matches that of the transistors’ E-B junctions.

An interesting by-product of this process is the bootstrap effect it causes in the amplifier.

When Q2 conducts less, the current through D2 conducts less as well. The control circuit increases the current through D2, but feeds Q8 as well. On the positive half-cycle of the audio, the load impedance on the collector of Q2 is extremely high, raising its gain considerably.

There is a trade-off here that one should be aware of.  This circuit produces another form of inaudible distortion. Because the load impedance on Q2 is so much higher on the positive half-cycle, and the regenerative amplifier (Q3 and Q5) feeds Q8, heavy loads on the amplifier will cause slight rounding of the negative peaks. Slightly lower gain under load would be normal in any amplifier of this type, but in this circuit, the positive peak doesn’t buckle as well as the negative peak, like in a more conventional design.

Also, the circuit can be simplified, depending on how much distortion you can tolerate in your application. Q7 can be deleted if a 32 ohm load is used. With a 32 ohm load, and Q7 present, no crossover distortion is visible, but only slightly visible with Q7 deleted. Also, Q1 can be eliminated, if a little more rounding of the negative peaks is tolerable. This is also barely detectable with a 32 ohm load.  Lowering the gain will also lower any distortion the circuit produces. The gain is determined by the ratio of R6 to R1 (10 in this case). R2 works with R6 to set the DC operating point (voltage on the emitter of Q8).

I used this circuit in a telephone headset amplifier without Q7 and it sounds just fine, but the whole system uses only 300uA from a 9-volt battery without audio passing through it. It has had the same battery in it (that was not new when I put it in) for about a year and it is still going strong.  The headset amp shuts off (to about 3uA consumption) after 2 minutes of inactivity.

More:  Audio Amplifier Circuits Telephone Circuits 

Got Designs?
Please eMail
if you want me to link to and/or post your original design Thanks.

HOME Schematics Index Hobby Corner Dave's Circuits Electronic Resources Contact Info
Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches

 About Us   |  Advertise on DiscoverCircuits.com   |   Report Broken Links  |    Link to DiscoverCircuits.com  |    Privacy Policy

Copyright  January, 1998 - November, 2017     David A. Johnson & Associates.  All Rights reserved. 

 Linking is ALLOWED but COPYING any content or graphics to your web site is EXPRESSLY PROHIBITED.