Amplifier circuit for high frequency signals, particularly for cable distribution systems, comprising at least a first transistor controlled at its base electrode by a signal source, and a difference amplifier

An amplifier circuit for high frequency signals, particularly for cable distribution systems, comprising at least a first transistor controlled at its base electrode by a signal source, and a difference amplifier in which the distortion produced across the base-emitter junction of the said transistor is eliminated in a simple manner by means of feerforward control.

The invention relates to an amplifier circuit for high frequency signals, 
particularly for cable distribution systems, comprising at least a first 
transistor controlled at its base electrode by a signal source, and a 
difference amplifier. 
The distortion produced in a given amplifier stage of an amplifier circuit 
may be reduced by applying negative feedback over this stage. As is known 
feedback pre-supposes simultaneous occurrence of input and output signals. 
In practice the signal applied to the relevant amplifier stage will need 
time to pass through the stage, the so-called delay time. Correction of 
the input signal is thus always effected after an incorrect output signal 
has been produced. This lagging of the error correction has a particularly 
large effect when the delay time is of the same order as the period of the 
signal, i.e. at high signal frequencies. 
A further known method of reducing the distortion produced in a given 
amplifier stage is to use so-called "feedforward" control as described in 
the article "A Feedforward Experiment Applied to an L-4 Carrier System 
Amplifier" by Harold Seidel, published in "I.E.E.E. Transactions of 
Communication Technology", Vol. com-19, no. 3, June 1971. 
As compared with the aforementioned feedback control feedforward control 
has the advantage that parasitic oscillation is impossible because there 
is no closed loop. In order to measure the distortion produced in the 
relevant amplifier stage the input and output signals have to be available 
simultaneously and with the same amplitude. In the known feedforward 
control methods this is achieved by on the one hand attenuating the output 
signal and on the other hand by delaying the input signal by the same 
amount as the attenuated output signal. Both signals are applied to the 
input of a difference amplifier which produces a signal representing the 
distortion produced in the amplifier stage. 
Particularly at high signal frequencies the distortion may be determined 
more accurately in this way than if feedback is used (where simultaneous 
occurrence of the input and output signals is impossible due to the 
non-compensated difference in delay between the two signals). 
The distortion measured is brought to its correct amplitude in the said 
difference amplifier and is then added to the output signal for the 
purpose of correction. 
The necessity in feedforward control of creating a distortion measuring 
point at which two signals are available both simultaneously and with the 
same amplitude leads to circuit arrangements which are difficult to 
realize in practice, particularly in amplifier circuits for a frequency 
range extending to very high frequencies. 
The invention provides a simple amplifier circuit in which feedforward 
control is used without using accurate, complicated circuits and to this 
end it is characterized in that the base-emitter junction of a transistor 
controlled at its base electrode by a signal source is directly connected 
in parallel with the input of a difference amplifier, while the output 
signal from said difference amplifier is applied to the output signal from 
the transistor with such an amplitude and phase that the distortion 
therein is substantially eliminated. 
The invention is based on recognition of the fact that the greatest 
distortion is produced in a transistor amplifier when the signal current 
passes the base-emitter junction of a transistor and is greater with 
larger signal currents. 
By directly measuring the distortion where it is produced, in this case at 
the base-emitter junction, and, subsequent to the measurement, converting 
said distortion directly into a correction signal, feedforward control 
suitable up to very high frequencies can be realized in a very simple 
manner. 
It should be noted that a distortion compensating circuit, fitted with 
tubes, is known from French Patent Specification No. 753,771. The 
occurrence of the distortion which is produced by a tube, can be 
determined by means of a voltage which is generated across a resistor, by 
the grid current of the tube. However, this voltage is no true 
representation of the character of the distortion so that also the 
compensation of the distortion cannot be perfect. 
The similarity to the circuit according to the invention is only apparent, 
because, as known, it is not possible with transistors to generate, with 
the base current a voltage across a resistor connected into the base lead, 
which voltage represents the distortion voltage produced across the 
base-emitter barrier layer.

FIG. 1 shows an amplifier circuit having input terminals 5 and 6 and output 
terminals 7 and 8. The base of a transistor 1 is connected to the input 
terminal 5, its collector is connected through a directional signal 
coupler 10 to the output terminal 7 and its emitter is connected through a 
resistor 3 to input terminal 6 and output terminal 8. The base and the 
emitter of the transistor 1 are also connected to the input of a 
difference amplifier 2. The output of this difference amplifier is 
connected to ground through the directional coupler 10 and a resistor 4. 
The load between the output terminals 7 and 8 is represented by a resistor 
9. 
The distortion occurring across the base-emitter diode of transistor 1 is 
directly applied to the input of the difference amplifier 2. The output 
signal from this difference amplifier represents the distortion in the 
collector-emitter current of the transistor 1. This output signal is added 
to the collector current of transistor 1 through the directional coupler 
10 in such a manner that the distortion is eliminated. The current through 
the load resistor 9 is thus free from that distortion which is produced as 
a result of the non linear base-emitter junction characteristic of the 
transistor 1. 
The directional coupler 10 ensures that there is no signal transmission 
from the output of the difference amplifier 2 to the collector of the 
transistor 1, and vice versa. 
With a correct choice of the resistance of the resistor 4 reflection which 
might otherwise occur on the output lead are prevented. 
FIG. 2 shows the alternating current circuit diagram of an amplifier 
circuit having input terminals 5 and 6 and output terminals 7 and 8. A 
signal source 15 having an internal resistance 16 is connected to the 
input terminals 5 and 6; a load resistor 9 may be connected to the output 
terminals 7 and 8. The input terminal 5 is connected to the base of a 
transistor 1 whose collector is connected to a supply terminal 23 and 
whose emitter is connected to the base of a transistor 21. The emitter of 
the transistor 21 is connected to the input terminal 5 through a resistor 
19. The emitter of the transistor 1 is connected to the output terminal 7 
through a terminating resistor 22. The terminating resistor 22 allows the 
correct termination to be obtained, which is achieved by choosing the 
value of this resistor to be equal to the value of the load resistor 9. 
The collector of the transistor 21 is also connected to the output 
terminal 7. 
Due to the operation of the transistor 1 as an emitter follower the voltage 
supplied by the signal source 15 gives rise to a signal current through 
the collector-emitter path of this transistor, the resistor 22 and the 
load resistor 9. This signal current produces a distortion voltage across 
the base-emitter junction of the transistor 1 due to the non-linear 
characteristic thereof. This voltage is applied to the series arrangement 
of the base-emitter junction of the transistor 21 and the resistor 19. As 
a result a collector-emitter current which will be referred to hereinafter 
as the correction current, is produced in the transistor 21, which current 
is similar to the distortion in the emitter current of transistor 1. By 
choosing the resistor 19 in such a manner that its resistance is equal to 
the terminating resistance 22 reduced by the value of the source resistor 
16, the amplitude of the correction current will be equal to the 
distortion current and both currents can compensate for each other. 
A distortion-free current thus results at the terminal 7 and is applied to 
the load. 
FIG. 3 shows an amplifier circuit in which corresponding components have 
the same reference numerals as those in FIG. 2, and which has input 
terminals 30 and 31 and output terminals 7 and 8. The base of a transistor 
35 is connected through a coupling capacitor 32 to the terminal 30. This 
base is also connected to a supply lead 23 through a resistor 33 and to 
ground through a resistor 34. 
The emitter of the transistor 35 is connected to ground through an inductor 
36 and a resistor 37 while the collector of this transistor is connected 
to the supply lead 23. The emitter of the transistor 35 is also connected 
to the base of the transistor 1 through the terminal 5. The collector of 
transistor 1 is connected to the supply lead 23 while its emitter is 
connected to the output terminal 7 through the resistor 22 and to the base 
of the transistor 21 through a capacitor 39. This base is also connected 
to the supply lead 23 through a resistor 40 and to ground through a 
resistor 41. 
The collector of the transistor 21 is connected to the terminal 7 while its 
emitter is connected to ground through a resistor 42 and an inductor 43 
and to the base of the transistor 1 through a series arrangement of a 
resistor 19 and a capacitor 38. 
A signal voltage applied through the coupling capacitor 32 to the base of 
the transistor 35 is passed on unchanged to the base of the transistor 1. 
The transistor 35 ensures that there is a low source impedance at the 
terminal 5. The d.c. bias of the transistor 35 is determined by the 
resistors 33 and 34 and the resistor 37. The inductor 36 ensures that the 
signal current in transistor 35 is sufficiently low. 
The signal voltage applied to the base of the transistor 1 is converted 
into a corresponding signal current through the resistor 22 and the load 
resistor 9. 
The distortion produced across the base-emitter diode of the transistor 1 
is eliminated by the correction signal generated by the transistor 21 in 
the same manner as that described above. 
As was noted in the description of FIG. 2 the value of the resistor 19 
should be chosen to be equal to that of the terminating resistor 22 
reduced by the source impedance at the terminal 5 so as to obtain the 
correct distortion compensation. 
Consequently the said source impedance must be lower than the value of the 
termination resistor 22, which is achieved by the emitter follower action 
of the transistor 35. 
It should be noted that the transistor 35 does not itself produce 
substantially any distortion because only a small signal current flows 
through it. 
FIG. 4 shows an amplifier circuit in which components corresponding to 
those in FIG. 3 have the same reference numerals. The paramount difference 
from the amplifier circuit of FIG. 3 is that the PNP-transistor 21 is 
replaced by an NPN-transistor 46. If the various resistances are correctly 
chosen the inductors 36 and 43 occurring in the amplifier circuit of FIG. 
3, the resistors 37 and 42 and the capacitor 38 may be omitted in the 
amplitude circuit of FIG. 4. 
The manner of distortion compensation in this amplifier circuit is 
otherwise the same as that described for the amplifier circuit of FIG. 3. 
FIG. 5 shows an amplifier circuit in which components corresponding to 
those in FIG. 4 have the same reference numerals. The difference from the 
previous amplifier circuit resides in the fact that the collectors of the 
transistors 35 and 1 are both connected to the output terminal 8 and to 
the supply lead 23 through a common collector resistor 45. This supply 
lead is connected to the output terminal 7. The load resistor 9 is 
arranged between the output terminal 7 and 8 i.e. in parallel with the 
collector resistor 45. 
The manner of distortion compensation in this amplifier circuit is the same 
as that employed in the previous amplifier circuit. Thus in this case too 
the circuit in which distortion current circulates due to the non-linear 
characteristic of the base-emitter junction of the transistor 1 is 
constituted by the collector-emitter path of the transistor 1, the 
resistor 22, the collector-emitter path of the transistor 46, the emitter 
resistor 19 and the collector-emitter path of the transistor 35. This 
means that the current through the collector resistor 45 is free from 
distortion so that the current through the load resistor 9 arranged in 
parallel therewith also does not include any distortion.