Floating power supply using dual npn transistor

Details Floating power supply using dual npn transistor Floating power supply using dual npn transistor

2000-10-18

2001-11-20

Lam, Tuan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific identifiable device, circuit, or system

With specific source of supply or bias voltage

C327S535000, C327S540000

Reexamination Certificate

active

06320452

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to power supply, particularly to a floating power supply for a voltage follower operating from a high voltage source.
(2) Brief Description of the Related Prior Art
A voltage follower usually uses an operational amplifier (OPA) with the input signal voltage applied to the non-inverting input of the OPA and the output of the OPA connected to the inverting input of the OPA as shown in FIG.
1
. Most integrated operational amplifier operates from a low voltage power supply. When the power supply is derived from a high voltage source, the high voltage source must be shifted down to a lower supply voltage for operating the OPA.
A prior art circuit for stepping down the supply voltage in operating the OPA
10
as a voltage follower is shown in FIG.
1
. The input voltage Vi is applied to the non-inverting input terminal of the OPA
10
. The output of the OPA
10
is connected to a complementary emitter follower with an npn transistor
122
and a pnp transistor
124
. The collector of the npn transistor
122
is connected to a high voltage 120V supply, and the collector of the pnp transistor
124
is connected to a high negative voltage −120V supply. The output voltage Vo is connected the common emitter of the npn transistor
122
and pnp transistor
124
. The base of the npn transistor
122
is clamped to the output voltage Vo through a Zener diode
147
which is biased by the high positive voltage +120V power source through the resistor
162
. Thus, the base of the npn transistor is biased with a voltage equal to Vi+Vz. The positive supply voltage V+ applied to the OPA
10
is then equal Vi+Vz−Vbe, which is a much lower voltage than the 120V high voltage source, where Vbe is the dc base-to-emitter typically around 0.7 V. Similarly, the negative supply voltage V− applied to the OPA
10
is equal Vi−Vz−Vbe, which is much lower in magnitude than the negative −120V high voltage supply.
Since the collector of the npn transistor
122
is connected to the 120V high voltage supply and if Vo centers around 0V, then the V
CE
across the npn transistor
122
is around 120V and requires a high voltage npn transistor. Similarly, the large V
CE
across the pnp transistor
124
requires a high voltage pnp transistor. Unfortunately, high voltage pnp transistors are not as readily available commercially as npn transistors. It is desirable not to use any high voltage pnp transistors. Another drawback is that the high voltage npn transistor
122
and the high voltage pnp transistor
124
should have matching characteristic. While it is easy to find matched npn transistors, it is difficult to find such a matched complementary pair.
SUMMARY OF THE INVENTION
An object of this invention is to provide a low voltage supply for a floating voltage follower from a high voltage source. Another object of this invention is to provide a low voltage supply for an operational amplifier operating as a voltage follower. Still another object of this invention is to provide a low voltage supply for an operational amplifier using all npn transistors.
These objects are achieved by connecting two npn transistors, two Zener diodes and a current source in series across a high voltage source. The input voltage plus a Zener reference voltage is applied to the base of transistor near the positive terminal of the high voltage source. Then a low positive supply voltage nearly equal to the input voltage plus the Zener voltage (Vi+Vz) is derived at the emitter. This low positive supply voltage is dropped by the two series Zener diodes to obtain a low negative supply voltage equal to Vi−Vz.


REFERENCES:
patent: 5418674 (1995-05-01), Davies et al.
patent: 5552746 (1996-09-01), Danstrom
patent: 6049234 (2000-04-01), Miyashita et al.

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