Wideband amplifier having separate outputs

A wideband amplifier having a direct output (C) to which is connected a level shifting stage providing at least one level shifted output. A first transistor emitter follower (Q.sub.3) is arranged in series with at least one level shifting element (Q.sub.4). The first transistor (Q.sub.3) has first resistor (R.sub.2) disposed between its base and a supply voltage source (V.sub.cc). A negative feedback circuit (R.sub.F1) is coupled to at least one level shifted output. The level shifting stage also includes at least second and third transistors (Q.sub.20, Q.sub.30) whose bases are connected to one said level shifted output (S) and which are arranged so that their collectors form separate first and second outputs (S.sub.1, S.sub.2) respectively. A fourth emitter follower (Q'.sub.3) is cascaded between the first emitter follower (Q.sub.3) and the level shifting element (Q.sub.4). A fifth transistor (Q.sub.10) forms a current mirror circuit with said level shifting element and has its main current path connected in series between the main current path of the first emitter follower (Q.sub.3) and a terminal of the level shifting element (Q.sub.4) not connected to the fourth emitter folllower (Q'.sub.3).

FIELD OF THE INVENTION 
The present invention relates to a wideband amplifier having one direct 
output to which is connected a level shifting stage presenting at least 
one level shifted output and comprising at least a first transistor 
emitter follower arranged in series with at least one level shifting 
element, said first transistor having a first resistor disposed between 
its base and a supply voltage source, and a negative feedback circuit 
originating from at least one level shifted output. 
An amplifier of this type is known from the article entitled "A 4-Terminal 
Wide-Band Monolithic Amplifier" by Robert G. Meyer and Robert A. 
Blauschild, published in IEEE Journal of Solid State Circuit Vol. SC 16, 
No. 6, Dec. 1981, pp. 634 to 638, more particularly, FIGS. 2 and 8. 
BACKGROUND OF THE INVENTION 
Such an amplifier is beneficial in having a wide passband as well as a low 
noise level and favourable values for the input and output impedances. 
It is an object of the invention to improve the characteristic features of 
an amplifier that has separate outputs so that a considerable separation 
between the outputs is obtained, for example, for realising a 
mixer-separator which has a television output and a video recorder output. 
SUMMARY OF THE INVENTION 
The basic idea of the invention consists of using an amplifier of the 
aforementioned type which intrinsically has interesting characteristics, 
and improving the characteristics of the voltage shifting circuit of the 
main negative feedback loop so that a proper isolation is obtained between 
separate outputs which are connected thereto. 
An amplifier according to the invention is thus characterized in that it 
comprises at least second and third transistors forming separate outputs, 
whose bases are connected to one said level shifted output and which are 
arranged so that their collectors form separate first and second outputs 
respectively, in that the level shifting stage comprises a fourth emitter 
follower cascaded between the first emitter follower and the level 
shifting element and in that it includes a fifth transistor forming a 
current mirror circuit with said level shifting element and whose main 
current path is connected in series between the main current path of the 
first emitter follower and a terminal of the level shifting element not 
connected to the fourth emitter follower. 
The current mirror circuit permits diminishing the output impedance of the 
shifting circuit and improving the isolation between the separate outputs 
without augmenting the noise level. 
According to an advantageous embodiment of the invention, the amplifier 
comprises a sixth transistor whose base constitutes an input of the 
amplifier and whose collector is connected to the base of a seventh 
transistor constituting a Darlington stage with an eighth transistor 
downstream in the circuit, said direct output being connected to the 
collector of the eighth transistor which is connected to the base of the 
first emitter follower. 
Said negative feedback circuit may then comprise a second resistor 
connected between said level shifted output and the emitter of the sixth 
transistor which has a third emitter resistor. A fourth resistor may be 
inserted between the emitter of the eighth transistor and the base of the 
sixth transistor so as to form an auxiliary negative feedback circuit 
permitting defining the input impedance without introducing a low-value 
input resistor which would cause noise. 
A current limitation in the main negative feedback loop may be obtained 
with a ninth transistor whose base is connected to said level shifted 
output and whose collector is connected through a fifth resistor to the 
base of a tenth transistor whose main current path is connected between 
said supply voltage source and, on the one hand, a fifth bias resistor of 
the collector of the sixth transistor and, on the other hand, the first 
resistor. 
A shift of four base-emitter voltages may be obtained by cascading an 
eleventh emitter follower between the first and fourth emitter followers, 
a twelfth transistor forming a current mirror circuit with the level 
shifting element, its main current path being connected in series between 
the main current path of the eleventh transistor and the terminal of the 
level shifting element not connected to the fourth emitter follower. 
A first current source may be connected to the terminal common to the 
fourth emitter follower and the level shifting element and at least one 
second current source may be connected to the base of at least one of the 
first and fourth transistors. At least one of said current sources may be 
constituted by a seventh resistor.

DESCRIPTION OF EMBODIMENTS 
As shown in FIG. 1, a wideband amplifier as described in the above 
publication by MEYER and BLAUSCHILD comprises a transistor Q.sub.1, whose 
base constitutes the input E of the amplifier and whose collector, 
connected to a supply voltage source V.sub.cc, is connected to the base of 
a transistor Q.sub.6 constituting the upstream transistor of a Darlington 
stage having a downstream transistor Q.sub.2. A resistor R.sub.3 is 
connected between the emitters of the transistors Q.sub.2 and Q.sub.6. The 
interconnected collectors of the transistors Q.sub.2 and Q.sub.6 
constitute a direct output C of the amplifier. A resistor R.sub.2 may 
connect them to the supply voltage source V.sub.cc. A main negative 
feedback loop which defines the gain of the amplifier comprises a 
two-level shifting circuit, that is to say, a first transistor Q.sub.3 
arranged as an emitter follower from output C, its collector being 
referenced to the supply voltage source V.sub.cc and cascaded with a diode 
Q.sub.4 realised here as a transistor of the same type as Q.sub.3 and 
whose base and collector are interconnected. A negative feedback resistor 
R.sub.F1 is connected in series between the cathode (emitter) of the diode 
Q.sub.4 and the emitter of the transistor Q.sub.1, which is connected to 
the common mode pole (ground) through a resistor R.sub.E1. 
The gain of the amplifier thus has the following value: 
##EQU1## 
The Darlington stage provides a large open loop gain. An additional 
negative feedback loop which favourably influences the input and output 
impedances comprises a resistor R.sub.F2 connected between the base of the 
transistor Q.sub.2 and the base of the transistor Q.sub.1 (input E). The 
resistor R.sub.F2 makes it possible to apply a D.C. bias current to the 
base of transistor Q.sub.1. 
Such an amplifier, comprising a main negative feedback loop and an 
additional negative feedback loop as required, has a wide passband and a 
relatively low noise level and is therefore suitable for amplifying weak 
signals such as aerial signals. 
FIG. 2 represents a diagram of a separator-mixer comprising an amplifier A 
receiving on its input aerial signals .psi. and having two separate 
outputs, one (TV) for feeding a television set and the other (VCR) for 
feeding a video recorder. These two outputs are to have a proper isolation 
with respect to each other and with respect to the aerial. 
In FIG. 3 the elements performing like functions shown in FIG. 1 have like 
reference characters. The output shifting circuit has three levels for 
shifting with three base-emitter voltages, two transistors Q.sub.3 and 
Q'.sub.3 being cascaded as emitter followers with a transistor Q.sub.4 
arranged as a diode. The emitter of the transistor Q'.sub.3 constitutes an 
output S whose level is shifted by two base-emitter voltages, and from 
which two separate output signals S.sub.1 and S.sub.2 are obtained in the 
following manner. Two transistors Q.sub.20 and Q.sub.30 have their 
collectors connected to the supply voltage source V.sub.cc through the 
respective resistors R.sub.20 and R.sub.30, their emitters connected to 
the common mode pole through the respective resistors R.sub.E20 and 
R.sub.E30 and their bases connected to the output S. The collectors of the 
transistors Q.sub.20 and Q.sub.30 constitute the respective separate 
outputs S.sub.1 (for example, VCR output) and S.sub.2 (for example, TV 
output). 
The isolation between the outputs S.sub.1 and S.sub.2 and the amplifier 
input or between themselves is not perfect. At high frequencies the 
collector-base capacitance of the transistors Q.sub.20 and Q.sub.30 
results in part of the signals on output S being reinjected and signals 
from one output being reinjected into the other. 
As observed above, for improving this isolation it is desirable to diminish 
the value of the output impedance of the voltage shifting circuit (point 
B) and also of point S and to augment the impedance of point A (base of 
transistor Q'.sub.3). For this purpose, a transistor Q.sub.10 has its base 
connected to point S and its main current path connected between the 
points A and B, that is to say, in a series combination with the main 
current path of the transistor Q.sub.3. If the transistors Q.sub.4 and 
Q.sub.10 are identical, the output current I.sub.3 is uniformly divided 
between the transistors Q.sub.10 and Q.sub.4. 
Let us assume that Z.sub.B1 and Z.sub.S1 are the output impedances at 
points B and S respectively, without transistor Q.sub.10, and Z.sub.B2 and 
Z.sub.S2 the output impedances at points B and S respectively, with 
transistor Q.sub.10. 
Let us assume that Z.sub.A1 is the input impedance at point A without 
transistor Q.sub.10 and Z.sub.A2 is the input impedance at point A with 
transistor Q.sub.10. We then have: 
##EQU2## 
when r.sub.d =emitter resistance of a transistor 
.beta.=current gain of transistor Q.sub.3. 
The level shifting (three base-emitter voltages of a transistor) remains 
substantially identical (less than 18 mV). 
The transistor Q.sub.3 (passed through by a current I/2) is fed by the 
transistor Q.sub.10. 
For optimization of the circuit the dynamic resistances of the diodes are 
to be very low relative to the value of R.sub.F1. This condition can 
easily be fulfilled with the proposed circuit which augments neither the 
current consumption nor the noise level. 
The addition of a transistor Q.sub.10 provides an improvement of up to 
about 10 dB for the isolation between the input and output of the 
amplifier, that is to say, between the aerial and one of the outputs 
S.sub.1, S.sub.2 as well as between the two outputs. 
By way of example the values of the components may be the following: 
______________________________________ 
R.sub.1 = 
500 .OMEGA. 
R.sub.2 = 
50 .OMEGA. 
R.sub.F1 = 
120 .OMEGA. 
R.sub.F2 = 
600 .OMEGA. 
R.sub.E1 = 
15 .OMEGA. 
R.sub.E2 = 
80 .OMEGA. 
R.sub.E6 = 
500 .OMEGA. 
______________________________________ 
In the following it will be shown how to limit the current I.sub.3 which 
passes through the main negative feedback resistor R.sub.F1. 
Let us assume that n is the number of levels of the voltage shifting 
circuit (for FIG. 3, n=3). 
As a first approximation we have: 
##EQU3## 
I.sub.3 is very sensitive to variations of V.sub.cc and of the 
temperature. 
In order that the negative feedback has a good linearity and a gain 
independent of the voltage shifting circuit, the resistor R.sub.F1 is to 
have a value which is much higher than the output impedance Z.sub.B2 of 
the voltage shifting circuit, that is: 
##EQU4## 
Because the value of R.sub.F1 is selected to be a function of a certain 
number of parameters corresponding to the required performance of the 
arrangement and because I.sub.3 is sensitive to variations of temperature 
and variations of the supply voltage, it is desirable to control its 
variations at least in the first instance so as to avoid a current 
consumption which is too high in extreme conditions. 
In order to realise this, the basic idea is to provide that the voltage 
V.sub.3 at point B in essence accounts for the current I.sub.3. The 
current I.sub.3 may be controlled by measuring the voltage V.sub.3. 
The base of a transistor Q.sub.8 is connected tothe point B through a 
resistor R.sub.9 which has a high value, and its emitter is connected to a 
supply voltage source (ground) through a resistor R.sub.E8. An integration 
capacitor C.sub.8 is connected between its collector and its base. Its 
collector is connected through a resistor R.sub.8 to the base of a ballast 
transistor Q.sub.7 whose collector is connected to the supply voltage 
source V.sub.cc, its emitter to a terminal of the resistors R.sub.1 and 
R.sub.2. A resistor R.sub.7 is connected between the collector and the 
base of the transistor Q.sub.7 so that it forms a divider bridge with the 
resistor R.sub.8. The resistor R.sub.7 is chosen so that it does not 
disturb the amplifier. The current passing through the resistor R.sub.7 
depends on the current I.sub.3 and the voltage V'.sub.cc on the emitter of 
the transistor Q.sub.7 tends to diminish when the current I.sub.3 tends to 
augment, whence the obtained control because I.sub. 3 depends on V'.sub.cc 
instead of V.sub.cc with this configuration. 
FIG. 4 represents a structure in which the linearity of the voltage 
shifting circuit is improved by adding current sources at the points A and 
S, which sources are constituted by the respective resistors R.sub.10 and 
R.sub.4. 
FIG. 5 represents an embodiment corresponding to the case where n=4. An 
emitter follower Q".sub.3 is cascaded in series between the transistors 
Q.sub.3 and Q'.sub.3 and the transistor Q'.sub.10. The transistor 
Q'.sub.10 forms a current mirror circuit with the diode Q.sub.4 and has 
its main current path arranged in series with that of the transistor 
Q".sub.3. This configuration may permit diminishing I.sub.3. 
The invention is not restricted to the embodiments described and 
represented. In particular, the represented transistors are of the npn 
type, which lend themselves best to an integrated realisation because they 
are faster than their homologous pnp transistors with which an amplifier 
according to the invention also could be realised.