Amplifiers – With semiconductor amplifying device – Including field effect transistor
Patent
1983-04-06
1985-08-27
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including field effect transistor
330298, H03F 316
Patent
active
045381178
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to methods for reducing the power dissipation in power transistors of the MOS=Metal Oxide Semiconductor type. The invention is not applicable to transistors of non-insulated gate structure e.g. bipolar types.
The power transistor is used for the purpose of controlling the current in an external load. The transistor acts as a variable resistance, therefore the current flowing will inevitably cause a certain heat to be dissipated. This heat is detrimental to the junctions of transistors, causing a faster deterioration, finally leading to malfunction. In order to dissipate more power, the transistor has to be made physically larger, therefore more expensive.
Therefore, one attempts to reduce the power dissipation in power-transistor devices for example by using fastswitching circuits, but these are complicated and cause many disturbances. Alternatively the cooling has to be improved with heat-exchangers or the so called heat sinks and even with forced air cooling.
All of these disadvantages are overcome in a simple way by this invention. The principle--using a resistor to reduce the dissipation in the transistor, and "move" it to the resistor--is well known. But not until the invention of the MOS-transistor has the road been opened to carry out simple control and dividing arrangements for the transistor and resistor-current.
SUMMARY OF THE INVENTION
The invention is based on the principle that, through the insertion of a "threshold-voltage" between the gates of two transistors Q.sub.1 and Q.sub.2, Q.sub.1 will always conduct full current while Q.sub.2 can be controlled, linearly and continously. Q.sub.1 can be controlled linearly and continously, if and when Q.sub.2 is cut off. This is combined with the known technique: insertion of a resistance, R.sub.1, in series with Q.sub.1 which reduces the dissipation in Q.sub.1. However, this result is a total resistance which is not lower than the value of R.sub.1, unless Q.sub.2 exists. When the device is to be used as a variable resistance the value of this resistance has to be decreased to zero ohms, which is performed by Q.sub.2. Its dissipation is reduced by the fact that a substantial part of the total current flows through Q.sub.1 and R.sub.1, especially when the voltage across the device is high. When the voltage is low, the dissipation in Q.sub.2 is low nevertheless.
This type of control, with a threshold voltage, is impossible to carry out with bipolar transistors, because the input base of Q.sub.1 would comprise a short circuit for the threshold voltage, also termed "bias" from a "constant-DC-voltage-generating network".
For the overall definition of the invention, reference can be made to the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
The modes of operation of this invention are illustrated in FIGS. 1-5.
FIG. 1 shows the configuration of the circuit, which can be seen to have external terminations identical to that of a MOS transistor, which the said circuit replaces.
FIG. 2 shows its insertion in a practical circuit, which in addition contains a power supply (PS.sub.1), a load (Z.sub.L) and a driver amplifier (F.sub.1).
FIG. 3 shows the output current/input voltage characteristic.
FIG. 4 shows the required bias circuit.
FIG. 5 shows the currents through the transistor during one half sine-wave cycle in the load resistor (Z.sub.L) in FIG. 2.
DESCRIPTION OF PREFERRED EMBODIMENTS
The principle of the circuit is shown in FIG. 1. It signifies that the transistor-function is divided between two transistors, Q.sub.1 and Q.sub.2, of which Q.sub.1 is connected in series with a resistor, R.sub.1. In addition there is a source of voltage, V.sub.B, connected between the gates, having a polarity preventing Q.sub.2 conducting before Q.sub.1 --to such an extent that Q.sub.2 does not begin to conduct before Q.sub.1 is conducting full current. The input control voltage may be applied to the gate of Q.sub.1 or Q.sub.2, to which being not important.
It may appear inconvenient to use four components instead of
REFERENCES:
patent: 3557092 (1971-05-01), Kubicz
patent: 4072908 (1978-02-01), Murari et al.
patent: 4382195 (1983-05-01), Pohl et al.
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