Double input buffer for track-and-hold amplifier

Details Double input buffer for track-and-hold amplifier Double input buffer for track-and-hold amplifier

2000-12-21

2001-11-27

Wells, Kenneth B. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Current driver

C327S560000

Reexamination Certificate

active

06323700

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an input buffer for a switched emitter follower-like track-and-hold amplifier comprising an input stage with an input transistor, a first diode, a cathode side of which first diode is connected to an emitter of the input transistor, a first current source between on the one hand the junction between the cathode of the first diode and the emitter of the input transistor and on the other hand a first supply voltage line, an anode of the first diode being connected to a track-and-hold controlled emitter follower.
Such an input buffer is known from U.S. Pat. No. 5,583,459. Input buffers described therein have a current source connected between the anode of the first diode and a second supply voltage. The presence of a diode is crucial to disconnect the input signal during the hold-mode.
It is known to embody the current source between the anode of the first diode and the second voltage supply with MOSFETs. A problem with such a current source embodied with MOSFETs is the nonlinear junction impedance thereof. When high input frequencies are used as input signals to this input buffer it is especially the nonlinear junction capacitance that plays a prominent role, at lower frequencies the nonlinear junction resistance also will play a role. The nonlinear junction impedance leads to distortion of the signal at the anode of the first diode. That distortion cannot be prevented by increasing the current through the current source since more current requires a proportionally larger current source with inherently larger nonlinear junction impedance. Furthermore a strict trade-off exists between available voltage headroom, noise and nonlinear output impedance of the current source.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an input buffer in which the trade-off between voltage headroom, noise and nonlinear output impedance has substantially decreased.
An input buffer according to the invention thereto is characterised in that the first current source is a non-switched constant current source, in that a second current source is connected between a collector of the input transistor and a second supply voltage, in that a second transistor comprising a control electrode, a current input electrode and a current output electrode is connected with the control electrode to the junction between the collector of the input transistor and the second current source, with the current input electrode to the second supply voltage and with the current output electrode to the anode of the diode.
Thereby it is achieved that a nonlinear output impedance of the second transistor does not substantially interact in the signal path. Any nonlinear current flows through the second transistor itself. A consequence of connecting the second transistor according to the invention is that the junction between the collector of the input transistor and the second supply voltage has an AC content that is attenuated relative to the input AC content by the gain of the second transistor. Consequently a good current source, also in terms of noise, can be selected as the second current source.
In a preferred embodiment of an input buffer according to the invention the second current source comprises a MOSFET.
Thereby it is achieved that a threshold voltage is available for V
GT
of that MOSFET.
In a further preferred embodiment of the invention the second transistor is a MOSFET.
Thereby it is achieved that the voltage at the anode of the first diode can reach up to the V
GT
of the second transistor. That is of advantage in situations where the second supply voltage is a low voltage.
The trade-off between voltage headroom, noise and nonlinear output impedance, known from the prior art, has substantially decreased in an input buffer according to the invention.


REFERENCES:
patent: 5583459 (1996-12-01), Sone
patent: 5654665 (1997-08-01), Menon et al.
patent: 5939907 (1999-08-01), Miyazaki

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