Low distortion, low noise, amplifier

A bipolar or field-effect transistor amplifier with very large dynamic range for use as a preamplifier in a radio receiver, optical link reciver, or the like. The amount of gain is approximately an integral number. Diode-connected transistors in the collector load circuitry of a gain-providing transistor cancel the distortion from the non-linear effects of the emitter-base junction of the gain-providing transistor at high input signal levels. The number of diodes corresponds to the amount of gain desired. To reduce the noise generated by the amplifier, the emitter of the gain-providing transistor has an inductor in series therewith and the collector load circuitry has an inductor therein, the ratio of the inductances substantially determining the gain of the amplifier.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to amplifiers in general and, more particularly, to 
low distortion amplifiers using distortion cancellation techniques. 
2. Description of the Prior Art 
In the radio art, it is generally the case that the useful sensitivity of a 
receiver is determined by the front end of the receiver--the mixer stage 
and preamplifier, if any. The useful sensitivity of any receiver has an 
upper and lower bound--defined here as the dynamic range of the receiver. 
The noise generated by the receiver (internal noise) determines the 
weakest signal that can be received, typically referred to as the minimum 
discernible signal or the minimum signal which produces a specified 
signal-to-noise ratio in the receiver's output signal. Conversely, the 
overload characteristics of the receiver determines the maximum received 
signal strength that can be received without a predetermined amount of 
distortion in the receiver output signal. It is generally considered 
desirable to have a receiver that has high sensitivity without overloading 
in high signal strength environments--i.e., a receiver with very wide 
dynamic range. 
To increase the sensitivity of a receiver, a preamplifier is added between 
the antenna and the mixer portion of the receiver. The preamplifier serves 
to boost weak input signals to overcome the internal noise of the 
receiver, allowing the receiver to receive weaker signals than without a 
preamplifier. However, should a strong signal enter the receiver, 
non-linearities in the transfer characteristics of the preamplifier will 
add distortion products to the amplified signal. Further, should some 
strong, unwanted, signals be present in the passband of the receiver, the 
desired signal will be corrupted by the unwanted signals due to the 
distortion of the preamplifier--typically referred to as cross-modulation. 
A remedy that will reduce the gain of the preamplifier when strong signals 
are present is adding automatic gain control (AGC, typically derived from 
within the receiver) to control the gain of the preamplifier. However, AGC 
may not be desirable in all situations, such as in rapidly changing signal 
strength environments or if the reduction in gain of the preamplifier is 
so much that the signal-to-noise ratio of the desired signal is no longer 
sufficient. Another approach is to use active elements and preamplifier 
circuit configurations which can tolerate large signals without 
significant distortion and still provide the desired gain with low noise. 
For example, field-effect transistors have very good overload 
characteristics, compared to bipolar transistors, when operating class A 
either with a common source (emitter) or gate (base) configuration. 
However, depending on the frequency of interest, the gain and noise 
characteristics of field-effect transistors may not be as good as bipolar 
transistors. 
Selection of the active devices to be used in preamplifier is especially 
difficult in ultra- and super-high frequency applications. For example, 
with cellular telephone systems where the operating frequency is 
approximately one GHz, silicon field-effect transistors do not have 
sufficient gain and low noise for use as receiver preamplifiers. Using 
gallium arsenide field-effect transistors instead of silicon devices will 
achieve the desired gain and noise as well as good overload 
characteristics, but suffer from relative high cost and the impracticality 
of integration with other circuit components, such as a mixer, which are 
typically formed on a silicon substrate for low cost. But for the 
relatively poor overload characteristics of bipolar transistors when 
operated conventionally, the ruggedness, low noise, low cost, high gain, 
and integratability of bipolar transistors would be a good choice for 
receiver preamplifiers. 
Generally, the foregoing is also true for optical receivers. Preamplifiers 
boost the signal from an optical detector, typically a PIN diode, prior to 
further signal processing. While the preamplifier is generally necessary 
for long-haul systems where the received signal energy is weak, in 
short-haul systems where the received signal is strong, the preamplifier 
may cause received signal distortion, making the system unusable. It is 
generally disadvantageous to have two types of receivers (or manually 
adjust one type) depending on optical signal level received. 
SUMMARY OF THE INVENTION 
It is one aspect of the invention to provide an amplifier having 
predetermined gain with low distortion at high input signal strengths. 
It is a further aspect of the invention to provide the low distortion 
amplifier with a low noise figure to provide very wide dynamic range for 
receiver preamplifier or the like in radio or optical receivers. 
These and other aspects of the invention are provided for generally by an 
amplifier disclosed herein, disposed in an integrated circuit, having an 
input port and an output port, for providing a predetermined voltage gain. 
Said amplifier is characterized by a gain transistor, with an input 
terminal and two output terminals, the input terminal coupling to the 
input port and a first one of the output terminals coupling to the output 
port; and, a means for generating a signal substantially equal to the 
distortion generated by the gain transistor, coupled between the first one 
of the output terminals of the gain transistor and a first power supply 
rail. The distortions created by the gain transistor are substantially 
reduced by the means. 
The above aspects of the invention may also be obtained generally by a 
method of amplifying signals with reduced distortion. The steps are: 1) 
amplifying the signals applied to the input port with a gain transistor, 
the gain transistor having an input terminal and two output terminals, the 
input terminal coupling to the input port and a first one of the output 
terminals coupling to the output port; and, 2) substantially canceling 
distortion from the gain transistor with means for generating a signal 
substantially equal to the distortion generated by the gain transistor, 
coupled between the first one of the output terminals of the gain 
transistor and a first power supply rail.

DETAILED DESCRIPTION 
The present invention reduces the amount of distortion generated by an 
amplifier, particularly at high input signal levels, while keeping the 
noise generated by the amplifier low. Briefly, this is shown in the 
exemplary embodiment of FIG. 2. As shown, a means for generating a signal 
substantially equal to the distortion generated by gain transistor 13, 
here a plurality of diode-connected transistors 16.sub.1 -16.sub.N 
serially coupled to the collector of a gain providing transistor 13, 
substantially cancel the distortion from the gain transistor 13. The ratio 
of the impedance of the load impedance 17 (here an inductor) to the 
impedance of degeneration impedance 15 (also an inductor) substantially 
determines the gain of the amplifier 4. It is preferable that the gain of 
the amplifier 4 be an integer and the number of diode-connected 
transistors 16.sub.1 -16.sub.N be substantially equal to the gain of the 
amplifier 4. 
In FIG. 1, a typical, exemplary, superheterodyne receiver 1 is shown in a 
simplified block diagram. Signals received by antenna 2 pass through a 
first bandpass filter 3 to the input of preamplifier 4. The filter 3 is 
typically used to suppress signals, having frequencies outside the desired 
frequency band, from reaching the preamplifier 4. This filter, along with 
filter 5, are important in blocking signals having frequencies near the 
image frequency response of the receiver 1. The signals from the output of 
preamplifier 4 pass through filter 5 to mixer 6 where the desired signal 
is mixed with a carrier signal from oscillator 7. The desired signal is 
translated in frequency both down and up by the carrier signal, one of 
which is selected by intermediate frequency (IF) filter 8 and amplified by 
IF amplifier 9. The amplified signal from amplifier 9 is demodulated (or 
mixed with another carrier signal, filtered, and amplified one or more 
times in multiple conversion superheterodyne receivers) by detector 10, 
the demodulated result (receiver output) being sent to utilization device 
11. 
As discussed above, the preamplifier 4 amplifies weak signals to a level 
which would otherwise be insufficient to overcome internal noise from the 
receiver and make the demodulated signal to the utilization device 11 
useful. However, adding a preamplifier decreases the maximum received 
signal level that can be tolerated before distortion reduces the quality 
of the receiver output, the demodulated result sent to the utilization 
device 11, below a useful level. In addition, the non-linearities in the 
preamplifier 4 can cross-modulate the desired, relatively weak, signal by 
strong adjacent signals applied to the input of the preamplifier 4. The 
cross-modulation of the desired signal can reduce the quality of the 
receiver output below a useful level--even though the signal strength of 
the strong adjacent signal is less than the maximum that can be tolerated 
in the single-signal case. 
A receiver utilizing as a preamplifier 4 the amplifier 4 shown in FIG. 2 
will typically be less susceptible to cross-modulation than receivers 
using conventional preamplifiers. The comparison is valid, for example, 
between the amplifier 4 in FIG. 2, implemented with bipolar transistors in 
a common emitter configuration, and a conventional amplifier implemented 
with bipolar transistors in a common emitter configuration. Similarly, the 
comparison is generally valid when the active devices are all field-effect 
transistors with similar device configuration. 
The amplifier 4 of FIG. 2 has an input port 10 coupled to a bandpass filter 
3, similar to that shown in FIG. 1, which suppresses received signals, 
from antenna 2, outside the desired frequency band of interest. Also shown 
is a DC biasing means 11, which will be discussed in more detail below, 
used to bias the amplifier 4 to the desired operating point. Capacitor 12 
serves as a DC blocking capacitor to prevent the bias currents from 
biasing means 11 from entering the filter 3. 
A gain providing transistor 13, here in a common emitter configuration, has 
the base thereof coupling to the input port 10. The collector of 
transistor 13 couples to the output port 14, which will in turn couple to 
filter 5 (FIG. 1). The emitter of transistor 13 couples to a degeneration 
impedance 15, here an inductor, which in turn couples to a common point 
and power supply return, here ground. It is noted that the input port 10 
and output port 15 are also referenced to ground. The value of the 
impedance 15 will be discussed in detail below. The collector of 
transistor 13 also couples to one end of a string of series-coupled, 
diode-connected, transistors 16.sub.1 -16.sub.N, the purpose of which will 
be discussed below. The other end of the diode-connected transistors 
16.sub.1 -16.sub.N couples to a load 17 (here an inductor) which, with the 
transistors 16.sub.1 -16.sub.N, forms the load for the gain transistor 13. 
The overall voltage gain of the amplifier 4 is substantially established by 
the ratio of the impedance of the load 17 to the impedance of degeneration 
impedance 15. Preferably, the load 17 and degeneration impedance 15 are 
inductors instead of resistors since pure inductors do not contain noise 
sources contributing to the noise figure of the amplifier. It is 
understood that resistors may be substituted or the load 17 and 
degeneration impedance 15, but the noise generated therefrom may be 
substantial, degrading the performance of the amplifier 4. It is further 
noted that the degeneration impedance 15 may be formed by using the 
parasitic inductance of the wiring between the emitter of transistor 13 
and ground. However, the parasitic inductance of the transistors 16.sub.1 
-16.sub.N and the wiring to the collector of transistor 13 must be taken 
into account when determining the inductance of load 17. 
It is believed that the diode-connected transistors 16.sub.1 -16.sub.N 
substantially cancel the signal distortions caused by the gain transistor 
13. For example, distortions caused by the non-linear characteristic of 
the base-emitter junction of transistor 13, and amplified by the 
transistor 13, are compensated for by the series of diode-connected 
transistors 16.sub.1 -16.sub.N. Hence, the number of diodeconnected 
transistors 16.sub.1 -16.sub.N should be approximately equal to the 
desired voltage gain of the amplifier 4. It is, therefore, preferable that 
the desired voltage gain of the amplifier 4 be an integral number, N. As 
such, the impedance of the load 17 should be N times the impedance of the 
degeneration impedance 15. It is also preferable that the electrical 
characteristics of the diode-coupled transistors 16.sub.1 -16.sub.N be 
substantially the same as the electrical characteristics of the gain 
transistor 13. Moreover, the current density in all the transistors 13, 
16.sub.1 -16.sub.N should be substantially the same. This may be achieved 
by scaling the transistors 16.sub.1 -16.sub.N so that the current 
densities therein are substantially the same as that in transistor 13. 
However, because the gain of transistor 13 is large, the size of all 
transistors may be the same with little resulting difference in current 
densities in each. 
The bias means 11 supplies a temperature compensated current to properly 
bias transistor 13. To provide the desired temperature compensation, 
transistor 18, diode connected via resistor 19, is preferably sized to 
have substantially identical current density therein as transistor 13 and 
is disposed in the same integrated circuit as transistor 13. Current from 
the means 11 passes through resistor 20 which serves to decouple the 
transistor 18 from the input port 10 of the amplifier 4. As a result, the 
current from current source 21 determines the substantially 
temperature-independent collector current in transistor 13. 
EXAMPLE 
The amplifier 4 has been constructed for use as a preamplifier with a 
voltage gain of 2(N=2) in a digital receiver operating at a receiving 
frequency of about 1 GHz. The typical component values are given below: 
transistors 13, 16, 16.sub.2, 18--1.5.times.96 .mu.m emitter size, f.sub.T 
.apprxeq.12 GHz. 
transistor 18--1.5.times.24 .mu.m 
impedance 15--5 nH 
impedance 17--10 nH 
resistor 19--4K .OMEGA. 
resistor 20--1K .OMEGA. 
current source 21--1 mA 
It is noted that field-effect transistors (FETs) may be substituted for the 
bipolar transistors shown. Further, while the polarity type of the 
transistors 13, 16.sub.1 -16.sub.N and 18 are shown here as NPN, PNP 
transistors may be used instead. 
Another use for the amplifier 4 of FIG. 2 is as an amplifier, used in 
conjunction with a transimpedance amplifier 25, for an optical link 
receiver 24, as shown in FIG. 3. Here, optical signals received from an 
optical transmitter (not shown) are converted into electrical signals by 
an electro-optical device 25, such as a PIN diode. The electrical signal 
is first converted from a current to a voltage by a transimpedance 
amplifier 26. The signal from transimpedance 26 amplifier may then be 
amplified by an amplifier 4 prior to further processing by a signal 
processor 27, such as a slicer for digital data or filtering and further 
amplification for analog signals. The output of the signal processor 27 is 
then applied to the utilization device 11. As in the case with the radio 
receiver 1 in FIG. 1, the gain, distortion, and noise contribution of the 
transimpedance amplifier 26 and amplifier 4 can define the ultimate 
performance of the optical receiver 24. For the receiver 29 to have the 
same sensitivity without amplifier 4, the gain of transimpedance amplifier 
26 must be increased, decreasing the bandwidth thereof. Hence, utilizing 
the exemplary amplifier 4 shown in FIG. 2 may improve the performance of 
the receiver 24 beyond that possible with just a transimpedance amplifier 
26. 
Having described the preferred embodiment of this invention, it will now be 
apparent to one of skill in the art that other embodiments incorporating 
its concept may be used. It is felt, therefore, that this invention should 
not be limited to the disclosed embodiment, but rather should be limited 
only by the spirit and scope of the appended claims.