Temperature stabilized common emitter amplifier

A common emitter amplifier having a resistor in series with its emitter and a resistor in series with its collector is temperature stabilized by providing a current source in parallel with the resistor in series with the collector. The current source provides additional emitter current to the transistor to decrease the transistors internal AC emitter resistance. Cascoded transistors are also used to improve the performance of the amplifier.

BACKGROUND OF THE INVENTION 
This invention relates, in general, to amplifiers and more particularly to 
a temperature stabilized common emitter amplifier. 
Typically a common emitter amplifier is temperature stabilized by adding a 
resistor in series with the emitter of the transistor. The gain of the 
amplifier can then be calculated by dividing the resistance of this 
emitter resistor into the resistance of the collector load resistor of the 
amplifier, provided that the emitter resistor is much greater than the 
internal emitter resistance of the transistor and the collector load 
resistor is much smaller than the internal collector resistance of the 
transistor. Of course the smaller the resistance of the resistor in series 
with the emitter, the larger the gain. If the resistor is made too small 
though then an undesired effect occurs. The internal emitter resistance of 
the transistor becomes a dominant factor in calculating gain, and as is 
well known, the internal emitter resistance is directly proportional to 
temperature. Therefore, it is desirable to keep the external resistor in 
series with the emitter large enough so that the internal emitter 
resistance does not assume an overbearing factor in controlling gain. The 
collector load resistor cannot arbitrarily be increased to increase the 
gain due to the low power supply voltage normally associated with 
integrated circuits. In addition, increasing the collector load resistance 
would change the quiescent operating current of the amplifier if voltage 
feedback is provided because the voltage feedback requires the quiescent 
output voltage to be the same for different values of collector load 
resistance. If the collector load resistance is increased for a given 
value of supply voltage, V.sub.CC, and of feedback voltage, then of 
course, the emitter current will be lower and, as a result, the internal 
emitter resistance will increase. One scheme widely used in integrated 
circuits to increase the gain of a common emitter amplifier is to replace 
the collector load resistor with a current source. This works well for 
many applications, however, such a scheme is not satisfactory for 
applications requiring closely controlled gain of the common emitter 
amplifier since gain is not too closely controlled when the collector load 
resistor is replaced by a current source. 
Accordingly, it is an object of the present invention to provide an 
improved common emitter amplifier. 
Another object of the present invention is to provide a common emitter 
amplifier that has temperature stabilization and closely controlled gain. 
Yet another object of the present invention is to provide a common emitter 
amplifier having a current source in parallel with the collector load 
resistor. 
A further object is to provide a common emitter amplifier, having a 
collector load resistor, with additional emitter current to decrease the 
effects, of variations of the internal emitter resistance of the 
transistor with temperature, on gain of the amplifier. 
SUMMARY OF THE INVENTION 
In carrying out the above and other objects of the invention in one form, 
there is provided an improved common emitter amplifier that is temperature 
stabilized and has a closely controlled improved gain. A transistor is 
connected as a common emitter amplifier having a resistor in series with 
its emitter and a collector load resistor. A current source is added in 
parallel with the collector load resistor to supply additional current to 
the emitter of the transistor thereby reducing the internal resistance of 
the transistor's emitter. 
In one exemplification, the current source is provided by two transistors 
cascoded to provide a high impedance in parallel with the collector load 
resistor. In addition the transistor amplifier can be cascoded to present 
a high output impedance, if desired. 
The subject matter which is regarded as the invention is set forth in the 
appended claims. The invention itself, however, together with further 
objects and advantages thereof, may be better understood by referring to 
the following detailed description taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The gain of a common emitter amplifier can be approximated by the following 
equation: 
EQU A.sub.V = R.sub.X /(R.sub.E + r.sub.e) 
where A.sub.V equals the voltage gain of the amplifier, R.sub.X equals the 
collector load resistance in parallel with the internal collector 
resistance of the transistor, R.sub.E is the resistance of an external 
resistor in series with the emitter of the transistor, and r.sub.e is the 
transistor's internal AC emitter resistance. The internal resistance of 
the transistor's collector is usually assumed to be so much larger than 
the collector's load resistor that R.sub.X is generally considered to be 
equal to the collector's load resistor. The internal resistance of the 
transistor's emitter can be represented by the following equation: 
EQU r.sub.e = kT/q I.sub.E 
where k equals Boltzmann's constant, T is temperature, q is the charge of 
an electron, and I.sub.E is the emitter current of the transistor. From 
the equation for the gain it can be seen that if the resistor in series 
with the emitter is decreased substantially, in order to cause the gain to 
be larger, the internal resistance of the emitter can become the dominant 
factor. However, it will be noted that the internal resistance of the 
emitter is directly proportional to temperature and therefore will vary as 
temperature varies. Since temperature is not always easily controlled and 
Boltzmann's constant is a fixed value and the charge of an electron is 
fixed, the only parameter left is the emitter current. As can be seen from 
the above equation, as the emitter current is increased the internal 
emitter resistance decreases. If the internal emitter resistance becomes 
small enough it will then have a minimum effect on the gain of the 
amplifier. 
Referring now to FIG. 1, a transistor 11 is connected as a common emitter 
amplifier. Resistor 12 is in series with the emitter of transistor 11 and 
couples the emitter to a common reference 13. Resistor 14 is a collector 
load resistor for transistor 11 and couples the collector to a potential 
appearing on line 16. The output of the amplifier appears on line 18 which 
is connected to the junction formed by the collector of transistor 11 and 
resistor 14. An input to the circuit appears on line 19 which is connected 
to the base of transistor 11. A current source 17 appears in parallel with 
resistor 14. The purpose of current source 17 is to pump additional 
current into transistor 11 in order to decrease the internal emitter 
resistance of transmitter 11. As noted hereinabove, the gain of a common 
emitter amplifier is less affected by temperature when the internal 
emitter resistance of the transistor is decreased. The gain of the 
amplifier can now be increased by reducing resistor 12 without sacrificing 
the temperature stability of the amplifier. Transistor 11 will not become 
saturated with the additional current supplied by current source 17 as 
long as the quiescent operating point of the transistor remains the same 
which, of course, can be done by voltage feedback or by input bias. 
In FIG. 2, the current source 17 of FIG. 1 has been replaced by a PNP 
transistor 21 and a bias source for transistor 21. The bias source 
includes diode 22 and resistor 23. Resistor 23 establishes a current flow 
through diode 22 while diode 22 provides a biasing voltage for transistor 
21. Although the biasing for transistor 21 has been shown as being 
supplied by a diode and a resistor combination is should be noted that it 
could be supplied by any convenient source. The remainder of the circuit 
of FIG. 2 is the same as the circuit of FIG. 1. An input signal applied to 
line 19 will forward bias transistor 11 and will be amplified by 
transistor 11. The amplified input signal will appear on output line 18. 
The additional current supplied by transistor 21 will substantially all 
flow into the collector of transistor 11 as long as the impedance of the 
load connected to line 18 is much higher than the impedance afforded by 
the collector of transistor 11. 
FIG. 3 illustrates an embodiment of the invention which is more suitable 
for being made in monolithic integrated circuit form. A transistor 26 has 
been cascoded with transistor 11 to provide a high output impedance for 
the amplifier thereby making the resistance of resistor 14 dominant at 
node 20. Also another PNP transistor has been cascoded with transistor 21 
so that the impedance of the current source provided by transistor 21 and 
24 will be high compared to the resistance of resistor 14. This is 
desirable so that the current source does not influence the gain of the 
amplifier. Two series diodes 31 and 32 have been added in series with 
diode 22 to provide bias for transistor 24. Two diode drops are provided 
between the bases of transistors 21 and 24 to ensure that the cascoded 
current source does not saturate. Bias for transistor 26 is provided by 
transistor 27 which is in series with transistor 28. The base of 
transistor 28 is tied to the same bias source as transistor 21. Transistor 
28 serves as a current source for transistor 27. The base of transistor 27 
is connected to the base of transistor 11 thereby providing one PN 
junction or one diode drop potential difference between the bases of 
transistor 11 and 26. This biases the cascode amplifier to prevent the 
amplifier from saturating. Transistor 29 has been added in series with 
input 19 to increase the input impedance. Although transistor 29 is not 
needed to improve the operation of the common emitter amplifier it may be 
required in applications requiring a high input impedance. This may be 
more so the case when the amplifier is used in a circuit having feedback 
connected to the input of the amplifier. If the input impedance is high 
then the feedback circuit will not be loaded down. The emitter of 
transistor 29 is connected to the bases of transistor 27 and transistor 11 
and supplies the input signal to the base of transistor 11. Transistor 34 
serves as a current source for transistor 29 and biases up the input 
transistor to prevent reverse current from flowing into the emitter of 
transistor 29. The bias for transistor 34 is provided by diode 33 
connected in series with resistor 23. The collector of transistor 29 is 
connected to the power source appearing on line 16. In applications 
requiring a low input impedance at the output of the amplifier, 
transistors connected in a Darlington configuration can be connected to 
line 18 to provide the low output impedance. 
The current supplied by the current source formed by transistors 21 and 24 
is intended to increase the emitter current of transistor 11. However, if 
the load connected to line 18 is low enough in impedance it may be 
possible for some of the current from the current source to flow into the 
load. This problem can be overcome by use of the Darlington configuration 
discussed hereinabove. The circuit arrangement illustrated in FIG. 3 can 
be adjusted to operate at the same quiescent or Q point as the circuit of 
a simple, common emitter amplifier. 
By now it should be appreciated that there has been provided temperature 
stabilization for a common emitter amplifier having a closely controlled 
gain. The gain of the circuit illustrated in FIG. 3 has been held between 
one and two percent variation over temperature. However, the gain can be 
held to under one percent variation by feeding additional current from the 
current source through the emitter of transistor 11. The circuit of FIG. 3 
not only gives an increased gain but is very closely controlled and is 
very stable temperature wise. 
Although the invention has been illustrated as having certain types of 
transistors, it will be understood that other types of transistors or 
semiconductor devices can be substituted to achieve the advantages of the 
present invention. 
Consequently, while in accordance with the Patent Statutes, I have 
described what at present are considered to be the preferred forms of my 
invention it will be obvious to those skilled in the art that numerous 
changes and modifications may be made herein without departing from the 
true spirit and scope of the invention, and it is therefore aimed in the 
following claims to cover all such modifications.