Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-12-03
2003-07-29
Tokar, Michael (Department: 2819)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S257000, C330S284000
Reexamination Certificate
active
06600372
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to differential pair attenuation and control circuits and techniques, and more particularly to an attenuator control circuit and a temperature compensation circuit for controlling and stabilizing operation of a differential pair attenuator.
DESCRIPTION OF RELATED ART
The bipolar junction transistor (BJT) differential pair is often used as the key element in an attenuator or automatic gain control (AGC) amplifier. The input signal, in the form of a current, is injected at the common emitters (emitters connected together) of the BJT differential pair. The output signal, also in the form of a current, is derived from the collector of one of the transistors. The difference in base voltages between the differential pair determines the ratio of output signal current to input signal current.
The attenuation function often needs to be linear in decibels (dB) and invariant with temperature and process variations. If the attenuator is to be “linear in dB”, then the collector current, referred to as I
C
, of one of the differential pair must vary (increase/decrease) exponentially with a linear change in control voltage. The collector current is constrained, however, to a maximum bias current, referred to as I
BIAS
, provided by a constant current sink coupled between the emitter terminals of the differential pair and ground. For some large positive value for the voltage differential between the base voltages of the differential pair, referred to as V
D
, the ratio of the output current to the input current is one-to-one (1:1). For a range of large negative V
D
, the current I
C
typically does vary exponentially with linear changes of V
D
. For small values of V
D
, however, the current does not respond exponentially. It is also noted that a thermal coefficient voltage, referred to as V
T
, gives the transfer function a temperature dependence. The thermal coefficient voltage V
T
is the voltage equivalent of temperature, where V
T
=kT/q, where “k” is the Boltzmann constant in joules per degree Kelvin, T is the temperature in degrees Kelvin (absolute scale), and “q” is the magnitude of the charge of an electron. Simply applying the input gain control voltage between the bases of the differential pair, therefore, does not result in a temperature independent, “linear in dB” response as desired.
SUMMARY OF THE INVENTION
An attenuator control circuit according to an embodiment of the present invention controls operation of a differential pair attenuator to provide linear in decibels (dB) operation. The differential pair attenuator includes first and second control input terminals that collectively receive a control voltage which is intended to control the attenuation of output current of the differential pair attenuator. In one configuration, an input current signal is injected at the common emitters of the differential pair and an output current signal is developed at the collector of one of the transistors. The output collector is coupled to a supply voltage through a resistor. The difference in base voltages between the differential pair determines the ratio of output signal current to input signal current. It is desired that the attenuation function be linear in dB and invariant with temperature and process variations. The differential pair attenuator alone, however, does not meet the desired attenuation function in certain circumstances, such as when the control voltage is small or large and positive. Also, the differential pair attenuator is dependent upon temperature and process variations.
The attenuator control circuit corrects for these deficiencies of the differential pair attenuator. The attenuator control circuit includes first and second transistors forming a control differential pair that is biased by a bias current. The control differential pair has first and second current paths and first and second current control terminals, where the first and second current control terminals are coupled to the respective first and second control input terminals of the attenuator differential pair. The attenuator control circuit also includes a current control circuit that sources a supply current to the first and second current paths of the control differential pair, where the supply current is approximately equal to the bias current. The attenuator control circuit also includes an amplifier that has an input coupled to the current control circuit and an output coupled to the second control terminal of the control differential pair. The amplifier controls current through the second current path of the control differential pair and attempts to maintain constant total current through the first and second current paths. In one embodiment, the constant total current through the first and second current paths is approximately the same as the bias current. Since the total current through the control differential pair is kept constant by controlling one current path of the control differential pair, the other current path of the control differential pair exhibits the desired exponential attenuation function. Since the control differential pair is coupled in parallel with the differential pair attenuator, the output current of the differential pair attenuator also operates according to the desired exponential attenuation function.
The current control circuit may include a bias current circuit and a current mirror. The current mirror has an input coupled to the bias current circuit and an output coupled to the first and second current paths of the control differential pair. In this manner, the current mirror applies the same current developed by the bias control circuit to the current paths of the control differential pair. In one embodiment, the amplifier is a non-inverting amplifier that has its input coupled to the output of the current mirror. In an alternative embodiment, the amplifier is an inverting amplifier that has its input coupled to the input of the current mirror. Operation is similar in either case. The attenuator bias current circuit may include a bias current sink and a third transistor, where the third transistor has a control terminal and first and second current terminals. The control terminal of the third transistor is coupled to the first current control terminal of the control differential pair. Also, the first current terminal is coupled to the input of the current mirror and the second current terminal is coupled to the bias current sink.
In a more specific embodiment, the first, second and third transistors are matched bipolar junction transistors having common emitters coupled to a bias current circuit that sinks approximately twice the bias current. Furthermore, the attenuator control circuit may include a temperature compensation circuit coupled between the control terminal of the third transistor and the first current control terminal of the control differential pair. The temperature compensation circuit is a suitable fixed bias voltage circuit that applies a temperature proportional voltage between the control terminal of the third transistor relative and the first current control terminal of the control differential pair. The temperature compensation circuit operates to counteract the temperature dependence of the attenuator control circuit and the differential pair attenuator.
In one embodiment, the temperature compensation circuit includes first and second differential-to-single-ended stages, each having a differential input and an output. A first polarity of the differential input of the first stage is coupled to a first polarity of the differential input of the second stage forming a feedback node. A reference signal is applied to a second polarity of the differential input of each of the first and second differential-to-single-ended stages. A temperature independent current sink is coupled to bias the first differential-to-single-ended stage and a temperature proportional current sink is coupled to bias the second differential-to-single-ended stage. Further, a current circuit is coupled to the output o
Intersil America's Inc.
Nguyen Linh Van
Stanford Gary R.
Tokar Michael
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