E00c062.PDF

BASIC CIRCUITS

1.5 V AF Amplifier

An optimised two-stage design

By Stephan Weber Stephan.weber@connect.com

The brief: design a tiny AF ampliﬁer powered by a single 1.5 V cell, yet

capable of driving an 8

Ω

loudspeaker. How can these requirements best

be met?

A high power output cannot be

expected of an ampliﬁer running on

a 1.5 V power supply. For example,

the quiescent output voltage of the

amplifier is ?

is used, entirely adequate for normal

indoor listening.

Two Ampliﬁer Stages

V CC = 0.75 V, and so,

unless an output bridge is used, the

maximum possible output swing U s

is only ±0.75 V. This is reduced to

±0.55 V by the saturation voltage of

the output transistors. If the battery

is no longer fresh, or is under load,

this might be further reduced to say

U s = ±0.4 or 0.5 V. The RMS output

power into an 8 Ω load is then

⋅

R g

Since both the output voltage and

the control voltages of the discrete

transistors are limited by the low

supply voltage, the output transi-

stors must be connected in com-

mon-emitter configuration

(Figure 1) . In many AF power ampli-

fiers with a higher supply voltage,

the well-known push-pull emitter-

follower circuit is preferred.

In this design, the maximum out-

put current is approximately 60 mA.

If the transistor has a current gain of

150, its maximum base current will

U o

U i

004108 - 13

Figure 1. In the common-emitter conﬁguration

the emitters of the transistors are connected to

a ﬁxed potential.

P = U s 2 / 2

≈

15 mW.

This is not especially loud, but, as

long as a high-efficiency loudspeaker

U o1

U o2

feedback

U i2

004108 - 14

004108 - 11

Figure 2. Schematic of the differential ampliﬁer.

Figure 3. Theoretical circuit of the 2-stage ampliﬁer.

Elektor Electronics

12/2000

⋅

BASIC CIRCUITS

optional

2mA2

negative feedback more difficult: each stage

inverts the signal, and so the overall effect is

non-inverting. However, for negative feedb-

ack we require an inverted signal. We have

two alternatives: either provide negative

feedback on each stage separately, or use a

differential ampliﬁer ( Figure 2 ). A differen-

tial amplifier has similar properties to the

common-emitter circuit, but also provides a

non-inverting input. The bases of the transi-

stors in the differential amplifier form its

inputs, the collectors its outputs.

The constant current I k (the sum of the two

emitter currents) determines the quiescent

input current, while the internal resistance of

the current source sets the common mode

rejection ratio. Setting the operating point is

made rather easier by the use of a differential

amplifier, since the base-emitter voltages of

the two transistors are compensated for.

T10

optional

500µA

BT1

4µ7

1V5

330p

5k1

10µ

500µ

220p

330p

1 mA5

50µ

004108 - 12

Theory and Practice

Figure 4. Practical ampliﬁer circuit.

The theoretical circuit in Figure 3 shows the

two-stage configuration with differential

ampliﬁer, common-emitter output stage, and

the provision of negative feedback using a

voltage divider at the inverting input to the

differential ampliﬁer.

A number of details of the theoretical cir-

cuit must be worked out in order to arrive at

the practical circuit of Figure 4 . Particularly

critical is the way the bases of the output

transistors are connected. The base voltage

of the upper transistor must be V CC – U BE , that

of the lower U BE . These conditions can only

be simultaneously satisfied for a narrow

range of supply voltages, and small variati-

A, too high for

the input to an amplifier. A pream-

plifier stage is therefore required.

With the use of negative feedback,

the noise performance of the ampli-

fier can be improved in comparison

to a one-stage design. An IC desi-

gner might employ three or four sta-

ges, but our discrete circuit needs to

be as compact as possible.

If we were to connect our com-

mon-emitter output stage directly to

an emitter-follower stage, the base

voltage of the emitter-follower would

be 2

1.4 V, hardly practical with

a one-cell power supply. A better

solution is to use a common-emitter

configuration in both the preampli-

ﬁer and output stages.

U BE ≈

Negative Feedback

The use of two common-emitter sta-

ges in a row makes the provision of

Construction tips:

distortion (THD). The same effect can be obtained by replacing

R6 by a transistor current source.

Choice of transistors

We have used types BC549C (NPN), BC559C (PNP) and BF245A

(FET). The most important characteristics are a high current gain

and a low saturation voltage. All the transistors used in the pream-

pliﬁer stage should be from the same series. The JFET should have

a threshold voltage of less than 1.5 V: hence our use of the A-type.

Other suitable devices include the BF244A and BF256A.

What can I use it for?

The ampliﬁer would be a good partner for the Midget MW Radio

described in the March 2000 issue of Elektor Electronics. Alterna-

tively, it could be used as an output ampliﬁer for a crystal set.

Another possibility is an audio test ampliﬁer: a test probe can be

connected to the input of the ampliﬁer and used, for example, to

check signal strength and frequency or look for distortion in other

audio circuits. A loudspeaker or headphones will of course have

to be connected to the ampliﬁer’s output. Remove the low-impe-

dance load from the output: then the open-loop gain will increase

and the distortion will fall. Then reduce C4 and C5 appropriately,

and the ampliﬁer will operate as a preampliﬁer for signals up to

about 1 MHz. If RF transistors are ﬁtted, a broadband HF ampli-

ﬁer operating up to 100 MHz can be constructed. And ﬁnally, as

long as only a modest volume is required, the circuit could be

used as a power ampliﬁer for a siren.

Can I use a 4 Ω loudspeaker?

Yes. The output transistors will need to be replaced by higher-

power devices, such as the BC327-40 and BC337-40.

How can I improve the sound quality?

In general, choosing a higher-quality loudspeaker and improving its

mounting will make more difference to the sound quality than

modiﬁcations to the circuit. However, the bass cut-off frequency

can be adjusted by changing the coupling capacitors. The quies-

cent current of the ampliﬁer stages can also be increased. This

increases the loop gain and therefore reduces the total harmonic

12/2000

Elektor Electronics

then be around 400

⋅

BASIC CIRCUITS

ons will have a large impact on the quiescent

current consumption. The collector resistors

in the differential ampliﬁer also have a great

effect on the quiescent current of the output

transistors.

In practice also, the operating point of the

differential ampliﬁer must be set to minimise

the effect of variations in temperature and

supply voltage on the circuit. The maximum

base current of the output transistors has

already been calculated as 400 µA, and so the

quiescent current in the differential ampliﬁer

will need to be around 1 mA.

One side of the differential ampliﬁer circuit

is duplicated in order to provide better cou-

pling between the two stages. Its outputs

(i.e., the collectors of T1 and T2) drive the

base connections of the two output transi-

stors.

The collector resistance is set using a cur-

rent mirror made from PNP transi-

stors T7, T8 and T9. Transistor T4 in

conjunction with FET T10 sets the

quiescent current of the differential

amplifier essentially independently

of other effects.

No special precautions are requi-

red to set the quiescent current of

the output stage. A value of 1 to

2 mA is adequate, and a reasonable

range is acceptable. An adjustment

can be provided by fitting a poten-

tiometer between the emitters of the

current mirror output transistors and

V CC . Negative feedback resistors R3

and R4 set the gain at approxima-

tely 5. Thanks to negative feedback,

the input impedance is approxima-

tely 50 kΩ, and so in practice the

total input impedance is determined

by R2 (plus the input impedances of

T2 and T10).

The remaining components stabi-

lise the operation of the circuit.

Ampliﬁers employing negative feed-

back often have a tendency to oscil-

late; here, with only two stages, it is

less likely. The Miller capacitors C4

and C5 provide additional safety;

their values are not critical. Capaci-

tors C1 and C2 isolate the amplifier

from DC offsets at the input and out-

put. A reservoir capacitor (say

4.7 µF) across the power supply is

essential. The series RC combination

(22

(004108-1)

Elektor Electronics

12/2000

+ 10 nF) at the output ensures

a minimum load even at high fre-

quencies, and thereby ensures the

stability of the ampliﬁer.

Ω

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