A small single battery cell AF amplifier able to drive a low
impedance 8W load speaker will be
presented. Although 3V or 6V batteries are available the 1.5V
operation has some advantages: The efficiency is higher for low
and medium sound levels and for the same battery size a 1.5V cell
has a much larger capacity which results in longer battery life.
Of cource you can not get much power from a 1.5V supply. If the
bias output voltage is ½VCC=0,75V, you may get
theoretically a maximum swing of ±0,75V. But due to saturation
voltages of the transistors and battery voltage drop only about VP=±0,45V
are realistic which is equivalent to 15mW. This sounds not so
much but is fully sufficient for normal room sound pressure level
if the efficiency of the loadspeaker is not too low. Because not
only the output voltage but also the intermediate transistor
voltages are limited by the supply voltage for optimum voltage
efficiency the output transistors should work in common emitter
configuration. This is quite different to AF power amps for
higher supply voltages where in most cases the common collector
configuration is prefered.
The maximum output current is IP=VP/RL=62,5mA.
If the transistor beta is B=150 then the maximum base current is
about 400µA. This is quite much for an amplifier input so a
preceding stage is very useful. IC designers normally would even
build a 3 or 4 stage amplifier (and e.g. a 20 transistor bias
circuit), but our PA should be as simple as possible. For maximum
power gain in a direct coupled (best for low component count)
amplifier the common emitter configuration is best suited, and a
CB or CC stage is much worse. At low voltages a CC also has the
disadvantage that the output voltage smaller than the input
voltage by VBE. If you want to drive a CE stage from a
CC stage you need an input voltage of 2·VBE»1,4V, which is nearly impossible form a
single cell supply. Two CE stages are much easier to connect, but
this makes feedback more difficult because the total amplifier
will be noninverting. A solution will be a differential pair
which has nearly the same characteristics like a single CE stage
but features an inverting and a noninerting input. A further
advantage is that the inherent temparature compensation makes the
biasing more stable. All this ideas will result in this 1st
schematic :
Figure 1 : Principal AF PA configuration
You will see the two stages and the feedback path formed by a
resistive divider, but some things need to be changed. Very
critical is the base drive of the output transistors. The upper
transistor need a base voltage of VCC-VBE
and the lower one needs VBE. Both conditions can only
be valid at a very small supply voltage range. Even small changes
in VCC will change the output stage bias current quite
dramatically. At high bias currents the battery will be
discharged very fast and at low bias levels the amplifier will
have large cross-over distortions. A much better circuit is show
in figure 2.
Figure 2 : The complete audio PA circuit
Because the maximum output stage base current is about 500µA the
differential pair is biased at 1.5mA. To improve the coupling and
biasing one part of the differential pair is doubled. Also the
load resistor is replaced by a current mirror with pnp
transistors. For biasing the output stage no additional circuitry
is needed because 1..3mA bias current is pretty good and some
changes can be tolerated. If you like you might introduce an
adjustment point by connecting a 50W
poti between the emitters of the current mirror output. The over-all
gain is set to VCL=5 via R3 and R4. Without feedback
you will get the open-loop gain which is much larger :
VOL » SDiff·RL·ß
» ICDiff/UT·RL·ß
» 25
Rin » 2ß/SCDiff
» 2UT/ICDiff·ß
» 10kW
With feedback the input resistance is about 50kW, so R2 dominates the total input impedance.
What Simulation Not Says
Oscillations are often a problem designing amplifiers. In this
design there are really critical because there are only two
stages. The Miller capacitances gives some margin. But also
important is decoupling VCC via a 4,7µF capacitor.
Because a real loadspeaker behaves not like a 8W resistances a series RC combination (22W+10nF) should be connected to the amplifier
output.
Figure 3 : Photo of the first prototype
Some hints on realization
What Transistors are best?
The FET should have a low IDSS and VTO (BF256A
or BF244A/BF245A). The bipolar transistors should have a high
beta (e.g. >250), so the BC549C (npn) and BC559C (pnp) are a
good choice. The output transistors also need a low saturation
voltage, so the BC327-40/BC337-40 are better. For SMT designs BC849C/859C
and BC808-40/818-40 are well suited.
What changes are needed for 3V?
The most important change is to set the output bias voltage to 1,5V
for large output swing. This is e.g. possible with a resistor
connected from the inverting input to ground.
How to get a better sound?
The optimization of the loadspeaker and its enviroment is more
important than any PA changes. The easiest way to improve the
amplifier is to increase the bias currents and to replace R6 by a
current source. For the actual circuit THD is about 3%.
What are the applications?
Maybe a AM radio or an measurement amplifier to hear
into other audio circuits. You can also use the amp as a pre-amplifier
or with other transistors as an broadband RF amplifer(e.g. with
BF224/324). Of course there are also apllications possible in
which no good sound quality is needed but high efficiency, like a
sirene.
Link : Basic Analog Course
1. Quo vadis Analog Electronics 2. Passive and Linear 3. Vector Calculations 4. Passive RC Filters 5. Filters of Higher Orders 6. Power Supply 7. Power Supply in Detail 8. Voltage Regulation 9. Transistors 10. Transistor Characteristics 12. Bias Point Setting 13. Precise Current and Voltage Sources 14. Transmission Lines |
To EE Practice
Partly published in ELEKTOR Vol. 9/2000.
If you need more information,
please send me an email!
To Top - To Homepage - To EE Software |
![]() |
Copyright © 2000 Stephan Weber. All rights reserved.
Stand: May 24, 2000.