Monolithic wideband amplifier

A Darlington-connected transistor pair is modified by the addition of a third transistor to form a three-terminal composite transistor wideband amplifier the current gain of which is approximately double that of a single transistor.

BACKGROUND OF THE INVENTION 
The present invention relates to amplifiers in general, and more 
particularly to a monolithic wideband amplifier. 
In a transistor amplifier, high frequency behavior is limited by diffusion 
capacitance which is intrinsic to the device, the predominant capacitance 
being that of the base-emitter junction. The small-signal current gain 
(h.sub.fe or .beta.) as plotted versus frequency remains fairly flat over 
a range of low frequencies, and then rolls off at about 6 decibels per 
octave as the frequency is increased beyond the beta cutoff frequency 
f.sub..beta., the frequency where the magnitude of current gain .beta. is 
3 decibels down from its low frequency value. The frequency at which the 
current gain decreases to unity is designated f.sub.T. 
Darlington composite transistors are useful as input amplifiers because 
they provide a high input impedance and they are operable at low biasing 
currents. The small-signal current gain at low frequencies is equal to 
.beta..sub.1 .beta..sub.2, or simply .beta..sup.2 where the two 
base-emitter junctions of the Darlington pair are substantially identical. 
In plotting the frequency response, the logarithmic value of gain rolls 
off at a rate of 12 decibels per octave as frequency is increased above 
the .beta.-cuttoff frequency point. In general, 9 or 10 decibels per 
octave is considered to be the maximum allowable for good amplifier 
stability. 
Associated with the transfer function of an amplifier and the frequency 
response thereof is the phase response of beta. For a single transistor 
amplifier, the phase shift ideally is zero at about 0.1f.sub..beta., 45 
degrees at f.sub..beta., and 90 degrees at about 10f.sub..beta.. For a 
Darlington-connected pair of transistors, the maximum phase shift over the 
frequency range is near 180 degrees because two base-emitter junctions are 
involved. This makes the design of wideband feedback amplifier circuits 
employing the Darlington configuration difficult or impossible because the 
well known Bode criteria for a stable condition must be satisfied to 
prevent instability or oscillation. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an improved wideband amplifier is 
provided wherein a Darlington-connected transistor pair is modified by an 
additional transistor. 
A Darlington-connected transistor pair includes a first transistor the 
emitter of which is connected to the base of a transistor of the same 
conductivity type. The collectors of the two transistors are connected 
together so that a three-terminal composite transistor is formed in which 
the collectors provide a first terminal, the base of the first transistor 
provides a second terminal, and the emitter of the second transistor 
provides a third terminal. 
A third transistor, the collector and base of which are connected together 
to form a diode, is connected across the base-emitter junction of the 
second transistor in the above-described Darlington-connected transistor 
pair. The addition of the third transistor lowers the current gain of the 
overall amplifier structure from about .beta..sup.2 to about 2.beta., 
while providing an effective f.sub.T which is 1.5 to 2 times that of a 
single transistor. Additionally, the gain rolls off at a 6 decibels per 
octave rate rather than the 12 decibels per octave rate of the 
uncontrolled Darlington configuration, and the corresponding phase 
response is free of the excessive phase shift of the uncontrolled 
Darlington configuration. With the stabilized response thus provided, such 
an amplifier is suitable for use as an amplifier with feedback. 
The modified Darlington amplifier preferably is fabricated using standard 
monolithic integrated circuit techniques so as to ensure matched operating 
characteristics of the constituent components. 
It is therefore one object of the present invention to provide an improved 
wideband amplifier. 
It is another object to provide a wideband transistor amplifier for which 
the current gain is substantially doubled over that of a single transistor 
amplifier. 
It is a further object to provide a wideband transistor amplifier 
characterized by a higher frequency at which the current gain reduces to 
unity. 
It is an additional object to provide a modified composite transistor 
amplifier wherein the current gain rolls off at 6 decibels per octave at 
high frequencies and wherein the phase shift characteristic is within 
stability limits. 
Other objects and advantages of the present invention will become apparent 
to those having ordinary skill in the art when taken in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION 
Referring to FIG. 1, a wideband amplifier comprises three transistors 10, 
12 and 14 of the same conductivity type connected to form a modified 
composite transistor having a collector terminal 16, a base terminal 18, 
and an emitter terminal 20. The emitter of transistor 10 is connected to 
the base of transistor 12, and the collectors of transistors 10 and 12 are 
connected together. Transistor 14, the collector and base of which are 
connected together to form a diode, is connected across the base-emitter 
junction of transistor 12. 
The operation of the FIG. 1 circuit is best understood in terms of 
mathematical relationships. It is assumed that transistors 10, 12 and 14 
are matched and therefore have substantially identical operating 
characteristics, and it is further assumed that one unit of collector 
current flows into transistor 12 to provide a basis for establishing the 
remaining current expressions shown. The DC current gain may be calculated 
as follows: 
##EQU1## 
For high frequencies, let 
##EQU2## 
where 
##EQU3## 
and s is the LaPlace transform operator. 
Then 
##EQU4## 
On a log-log gain versus frequency plot, the approximate 2.beta. value 
calculated in equation (1) remains constant as frequency is increased to 
infinity; and the results of equation (2) produces a 45.degree. sloping 
line depicting that the gain rises from .beta.=1 at frequency f.sub.T to 
infinity at DC, or zero frequency. These two equations provide a piecewise 
linear approximation of the amplifier frequency response, and the point 
where the lines cross is the pole frequency, or .beta.-cutoff frequency 
f.sub..beta.. 
From the foregoing mathematical approximations, the overall frequency 
response may now be calculated for the modified composite transistor by 
letting 
##EQU5## 
where 
##EQU6## 
and T.sub..beta. =.beta..sub.o T.sub.T and substituting for .beta. in 
equation (1): 
##EQU7## 
A plot of the current gain versus frequency as derived from equation (3) 
is shown as the solid line in FIG. 2, and depicts the frequency response 
of the modified composite transistor. Also shown in FIG. 2 in dashed lines 
for purposes of comparison are the frequency response curves for a single 
transistor and for a Darlington composite transistor. 
Referring the response curves of FIG. 2, the response for a single 
transistor is depicted by a standard textbook plot wherein the current 
gain is -3 decibels (dB) at the pole frequency f.sub..beta. and rolls off 
at 6 dB/octave until unity gain is reached at f.sub.T. For a Darlington 
composite transistor, which transistors 10 and 12 alone would form, the 
small-signal current gain is equal to .beta..sup.2 +2.beta. for matched 
transistors, or approximately .beta..sup.2 where .beta. is large. In the 
response curve, there are two poles at frequency f.sub..beta. and a zero 
at about 1/2 f.sub.T, resulting in the break points shown. In the response 
curve for the modified composite transistor depicted by the solid line, 
there are poles at f.sub..beta. and about 1/2 f.sub.T and a zero at 2/3 
f.sub.T. Parameters such as effective f.sub.T and phase shift may be 
determined from equation (3), which is converted from the s domain to the 
more familiar complex frequency form by use of LaPlace transforms. The 
effective f.sub.T has been calculated to be 1.554 times the f.sub.T of a 
single transistor. Maximum phase shift at about 1/2 f.sub.T has been 
calculated to be -98.2 degrees, or about 8 degrees excess phase shift at 
1/2 f.sub.T. 
Thus it can be discerned that for the composite transistor of the present 
invention not only is the current gain substantially twice that of a 
single transistor and the effective f.sub.T thereof 1.5 to 2 times that of 
a single transistor, but the corresponding phase response is free of the 
excessive phase shift of the uncontrolled Darlington configuration. Such a 
modified composite transistor preferably is fabricated using standard 
monolithic integrated circuit techniques so that the transistors 10, 12 
and 14 may be carefully matched to ensure substantially identical 
operating characteristics. 
FIG. 3 shows the modified composite amplifier of FIG. 1 in a feedback 
amplifier embodiment. The emitters of transistors 12 and 14 are connected 
to ground. The collectors of transistors 10 and 12 are connected to the 
emitter of a transistor 30, which is operated as a common-base amplifier. 
The collector of transistor 30 is connected through a load resistor 32 to 
a source of positive voltage V.sub.cc. A feedback resistor 34 paralleled 
by a speed-up capacitor 36 is connected between the collector of 
transistor 30 and the base of transistor 10. A bias current cource 40 and 
a signal current generator 42 are also connected to the base of transistor 
10 to provide operation of the circuit. The output voltage signal is taken 
from the collector of transistor 30. A feedback amplifier of this 
configuration has been fabricated and tested, and stable operation of a 
frequency range of DC to 50 megahertz has been observed. 
While I have shown and described preferred embodiments of my invention, it 
will be apparent to those skilled in the art that many changes and 
modifications may be made without departing from my invention in its 
broader aspects. The appended claims thereof cover all such changes and 
modifications as fall therewithin.