Balancing Amplifier

The invention relates to a balancing amplifier comprising a pair of transistors arranged as a differential amplifier, the control electrode of one transistor receiving the signal to be balanced.

In order to realize a balancing amplifier, i.e. an amplifier which, starting from a signal applied to its input, produces two signals of the same amplitude and of opposite phase at its output, it is known to employ a differential amplifier. One input of said amplifier receives the signal to be balanced, and the other input is connected to a source of fixed potential. The input signal causes current variations of opposite sense in the load circuit of the transistors of the pair.

A drawback of these differential amplifiers is that they supply output voltages which are distorted relative to the input signal. This distortion appears when the relevant voltage between the control electrodes of the transistor pair exceeds some tens of millivolts. In order to eliminate this drawback, it has been proposed to include resistors in the emitter circuits of the transistors when they are of the bipolar type. Thus the distortion is actually reduced, but the amplifier gain is also reduced.

The invention provides a balancing amplifier which combines a low distortion with an acceptable gain.

To this end a balancing amplifier in accordance with the invention is characterized in that in each of the load circuits of the transistors of the pair there is provided a transistor arranged as a voltage follower in order to supply, by means of a negative feedback network, a negative feedback voltage to the control electrode of the transistor to which it is connected when the control electrode receiving the signal to be balanced is connected to the input terminal via an input impedance.

The invention will be explained by means of the following description taken with reference to the accompanying drawings, in which:

FIG. 1 shows the circuit diagram of the balancing amplifier in accordance with the invention;

FIG. 2 shows a variant of the balancing amplifier in accordance with the invention;

FIGS. 3a, 3b and 3c show in detail how the voltage dropper device employed in the amplifier shown in FIG. 2 can be realized.

The balancing amplifier shown in FIG. 1 employs bipolar transistors. Two of these transistors, designated by the reference symbols T.sub.1 and T'.sub.1 respectively, are arranged as a differential amplifier. The emitter of these transistors are connected together to a circuit 1 which presents a high impedance to variations of the emitter currents of transistors T.sub.1 and T'.sub.1. This circuit 1 is generally constituted by a current generator which supplies a current I.sub.o. In order to connect the collectors of the transistors T.sub.1 and T'.sub.1 to a supply voltage source supplying a voltage V.sub.cc, there have been provided two resistors 2 and 3 which respectively have a value R.sub.c and R'.sub.c. The control electrode of the transistor T.sub.1, its base, receives a signal v.sub.e which is applied to the input terminal 4 of the balancing amplifier, whereas the base of the transistor T'.sub.1 is connected to ground via a capacitor 5. Thus, the base of transistor T'.sub.1 is connected to a fixed potential for potential variations which are rapid relative to for example thermal drift.

The signals v.sub.s and v'.sub.s representing the input signal v.sub.e appear in phase opposition at the output terminals 6 and 7. This may be written as:

where A is the amplifier gain.

In accordance with the invention the balancing amplifier is characterized in that in each of the load circuits, which in the present example are constituted by the resistors 2 and 3, there is provided a transistor T.sub.2 and T'.sub.2 respectively, arranged as a voltage follower for supplying, via resistors 8 and 9 respectively having the values R.sub.B and R'.sub.B, a negative feedback voltage which appears at the emitters of the transistors T.sub.2 and T'.sub.2 when the control electrode of the transistor T.sub.1 is connected to the input terminal 4 via a resistor 10 having the value R.sub.A.

The collectors of the transistors T.sub.2 and T'.sub.2 are brought to a potential V.sub.cc. Via the resistors 12 and 13 of the value R.sub.E and R'.sub.E, respectively, the emitters of the transistors are connected to a negative supply voltage source supplying a voltage -V'.sub.cc.

In order to realise this amplifier as an integrated circuit, it is to be noted that said resistors R.sub.E and R'.sub.E are preferably replaced by transistors connected as a current generator.

Such a balancing amplifier operates as follows.

First of all the behaviour of the circuit for direct current is considered and in that case no allowance is made for the branch formed by the resistor 10 connected between the terminal 4 and the base of the transistor T.sub.1.

Since the transistors T.sub.1 and T'.sub.1 are arranged as a differential amplifier, their collector currents I.sub.c1 and I'.sub.c1 may be represented by: ##EQU1##

In these formulas:

I.sub.o is the current supplied by the current generator 1, g is the reduced transconductance, the normal transconductance G being related to said reduced transconductance by: ##EQU2## V.sub.B and V'.sub.B are the base voltages of the transistors, and V.sub.off is the offset voltage.

The collector potentials of the transistors T.sub.1 and T'.sub.1, V.sub.c1 and V'.sub.c1, and the emitter potentials of the transistors T.sub.2 and T'.sub.2, V.sub.E2 and V'.sub.E2, are written as:

In these formulas:

V.sub.BE2 and V'.sub.BE2 are the voltages between the bases and emitters of T.sub.2 and T'.sub.2.

When the voltage drops caused by the resistors R.sub.B and R'.sub.B, through which the currents I.sub.B and I'.sub.B flow, are considered, the following may be written: E1 ? ? ##STR1## where .beta. and .beta.' respectively represent the current gain of transistors T.sub.1 and T'.sub.1.

With the aid of these formulas the relationship between the quantity V.sub.E2 -V'.sub.E2, representing the balancing of the circuit arrangement and its parameters, is determined; this yields: ##EQU3##

In this formula: ##EQU4##

This formula is now compared with that relating to a balancing amplifier in which the resistors R.sub.B and R'.sub.B are not included and the bases of transistors T.sub.1 and T'.sub.1 are connected to ground; this yields: ##EQU5##

The advantages of the balancing amplifier in comparison with a conventional differential amplifier are apparent from equation (9).

The influence of the potential differences V.sub.BE2 and V'.sub.BE2 on the output voltages is reduced as K increases. It is to be noted that the adverse effect of the offset voltage V.sub.off on the output voltages is reduced by a factor equal to 1+K+.tau..

Now the gain of such an amplifier for variable signals will be examined. First the amplifier will be considered without the resistors 8 and 9. This amplifier consequently has a gain K. Subsequently, it is found that the balancing amplifier in accordance with the invention can be obtained from the preceding amplifier by adding the feedback resistor R.sub.B and the resistor of the value R.sub.A. The resistor of the value R'.sub.B has no effect on the gain for variable signals because the capacitor 5 brings the base of transistor T'.sub.1 to ground potential for variable signals. Thus, the gain A of the balancing amplifier in accordance with the invention is: ##EQU6##

and if R.sub.B /R.sub.A =10 and K=+100 this yields: A=-9.9.

Since it is assumed that the distortion of the balancing amplifier is attributable to a value of K which depends on the amplitude of the input signal, the gain A depends almost solely on the values R.sub.B and R.sub.A.

In order to improve the behaviour of the balancing amplifier in accordance with the invention at high levels, it is desirable to include voltage dropper devices 20 and 21 in the emitter circuits of the transistors T.sub.2 and T'.sub.2, which is shown in FIG. 2 in which corresponding elements bear the same reference numerals as in FIG. 1. Thus, a collector-base voltage for the transistors T.sub.1 and T'.sub.1 is obtained which is such as to ensure operation of these transistors in the linear portion of their characteristics.

FIGS. 3a, 3b and 3c represent the voltage dropper devices suitable for the balancing amplifier in accordance with the invention.

The device shown in FIG. 3a comprises a resistor 30 shunted by a capacitor 31, while that of FIG. 3b is constituted by a zener diode 32. FIG. 3c shows another example of a voltage dropper device. This device is constituted by an assembly of normal diodes 34 connected in series. In certain cases a single diode may suffice.

Although a balancing amplifier has been described which employs only bipolar transistors, it is evident that field-effect transistors may be used without departing from the scope of the invention.