Variable gain amplifier
AssigneeMatsushita Elec. Ind. Co. Ltd.
InventorMitsuru Tanabe
Life of this patent
Estimating the term…
Abstract
It is an object of the present invention to provide a variable gain amplifier of which impedance does not change when gains are switched. A transistor is turned on in a high-gain state and transistors are turned on in a low-gain state to switch between the gains, the area of the transistor is made equal to the area of the transistor to keep the same output load conditions and the same output impedance both in the high-gain state and the low-gain state. While the input impedance of a transistor becomes high because the transistor in a high-gain path is turned off in the low-gain mode, current passes through a transistor to lower the impedance of the collector of a transistor and, as a result, the input impedance is kept the same both in the high-gain state and the low-gain state.
Claims5
A variable gain amplifier in which an output circuit of a second transistor 2 is provided between an output circuit of a first transistor and a load, and an input signal provided to an input of the first transistor is amplified, the amplified signal is taken from a connecting point between said load and the second transistor, and the second transistor is turned on and off to switch between gains, wherein:
- a series circuit of an attenuator means and an output circuit of a third transistor is connected in parallel with an output-circuit of the second transistor;
- an output circuit of a fourth transistor is connected in parallel with a series circuit of the output circuit of the second transistor and said load;
- selector means is provided for switching between a high gain state and a low gain state;
- and
- said selector means turns off the third and fourth transistors and turns on the second transistor to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load and magnitude of the load, from said connecting point in the high-gain state, and turns off the second transistor and turns on the third and fourth transistors to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load and magnitude of the load, from said connecting point in the low-gain state.
A variable gain amplifier in which an output circuit of a second transistor 2 is provided between an output circuit of a first transistor and a load, and an input signal provided to an input of the first transistor is amplified, the amplified signal is taken from a connecting point between said load and the second transistor, and the second transistor is turned on and off to switch between gains, wherein:
- a series circuit of an attenuator means and an output circuit of a third transistor is connected in parallel with an output circuit of the second transistor;
- an output circuit of a fourth transistor is connected in parallel with a series circuit of the output circuit of the second transistor and said load;
- selector means is provided for switching a high gain state, a medium gain state, and a low gain state;
- the third, fourth, and fifth transistors are turned off and the second transistor is turned on to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load and magnitude of the load, from said connecting point in the high-gain state;
- the second and fifth transistors are turned off and the third and fourth transistors to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load and magnitude of the load, from said connecting point in the medium gain state;
- and
- the second and third transistors are turned off and the fourth and fifth transistors are turned on to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load and magnitude of the load, from said connecting point in the low-gain state.
Dependencies are inferred from the claim text.