Transistor amplifier that accommodates large input signals

Details Transistor amplifier that accommodates large input signals Transistor amplifier that accommodates large input signals

2000-09-29

2002-06-11

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S254000

Reexamination Certificate

active

06404285

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a gain stage of a differential amplifier used in a communication receiver at an intermediate frequency. Specifically, a gain stage is described which accommodates large differential input signals while maintaining high linearity.
BACKGROUND OF THE INVENTION
A differential amplifier, as seen in
FIG. 1
, is effective at amplifying signals having a low differential, voltage swing. Differential voltages, within a limited voltage and frequency range, applied to the inputs of the amplifier will be linearly amplified to produce a larger differential voltage at the collector outputs
26
,
27
. The collectors of transistors
20
,
21
are connected to a source of voltage VDD by resistors
23
and
24
, respectively. The emitters are connected to a current source
25
.
FIG. 2
is illustrative of the voltage gain property provided by the differential amplifier of FIG.
1
. The differential output voltage appearing at the collectors of transistors
20
and
21
, for a given differential input voltage, is plotted along the vertical axis and the differential input voltage is plotted along the horizontal axis. For differential input voltages between −Vt and Vt, the differential output voltage is substantially linear. Vt is the product of Boltzmann's constant, k, and the device absolute temperature, T, divided by the elementary charge, q, and is approximately equal to 0.0259 volts at room temperature. As may be seen from
FIG. 2
, dynamic range of the amplifier is defined to be the largest input signal for which the output is not yet clipped, i.e. +/−2Vt.
Large differential voltages applied to the differential amplifier cause distortion in the output signal. For differential input signals having absolute values increasingly greater than Vt, the voltage amplification of the differential amplifier becomes increasingly non-linear, as illustrated in FIG.
2
. Differential input voltages having absolute values exceeding 2Vt produce non-linear, differential output voltages that are clipped at a maximum absolute value. The collector currents of the two transistors,
20
and
21
, operate independently when the absolute value of the differential input voltage exceeds 2 Vt, since all of the amplifier's current is flowing in only one transistor.
In some circuit applications, it is desirable to produce a substantially linear amplification of differential input signals outside the domain of −Vt to Vt. However, this is not possible with prior art differential amplifiers of the type illustrated in FIG.
1
.
SUMMARY OF THE INVENTION
The differential amplifier of the present invention produces a substantially linear, differential output signal for a differential input signals exceeding ±2Vt without producing a clipped output signal. Increasing the dynamic range of the input signal is accomplished by connecting a regulation element with a pair of transistors forming a differential amplifier. Each transistor of the pair is connected to a first terminal of a power source through a separate impedance element. A current source connects the pair of transistors to a second terminal of the power source. The regulation element is connected between the first terminal of the power source and the current source and has an input connected to base terminal of the differential amplifier transistor.
A gain stage of this type reduces the gain of a differential amplifier without affecting the input and output impedances of the amplifier circuit, but ensures that a portion of the tail current flows in the load resistors even when the input differential signal reaches ±2Vt thereby significantly increasing the dynamic range of the source.


REFERENCES:
patent: 4423385 (1983-12-01), Evans
patent: 4612513 (1986-09-01), Seevinck
patent: 5883539 (1999-03-01), Kimura
patent: 6137362 (2000-10-01), Dufossez

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