Double switched track-and-hold circuit
A double switched track-and-hold circuit includes an input buffer having a first switched amplifier circuit for passing an input signal in a track mode and isolating the input signal from a buffer output in a hold mode, and a track-and-hold core circuit responsive to the input buffer and having a second switched amplifier circuit for tracking the input signal in a track mode and isolating the input signal from a track-and-hold output in a hold mode. The first and second switching amplifier circuits turn off approximately simultaneously during the hold mode to provide enhanced isolation.
1. A high speed double switched track-and-hold circuit operable at sampling bandwidths greater than 10 GHz comprising:
a track-and-hold core circuit including a switched amplifier circuit for tracking the input signal in a track mode and isolating the input signal from a track-and-hold output in a hold mode; and;
an input buffer amplifier including:
a differential amplifier stage,
an additional switched amplifier circuit comprising a switchable emitter follower buffer coupled to the output of the differential amplifier stage and the track-and-hold core circuit,
a clamping circuit coupled to outputs of the switchable emitter follower buffer configured to clamp a signal at the output of the input buffer amplifier during the hold mode and configured to divert current from the switchable emitter follower buffer in the hold mode, and
the additional switched amplifier circuit in combination with the clamping circuit configured to provide minimal input capacitive loading, low output impedance in the track mode, and high output impedance in the hold mode to enable sampling bandwidths greater than 10 GHz and simultaneous switching of the track-and-hold core circuit and the switchable emitter follower to provide enhanced isolation.
2. The double switched track-and-hold circuit of claim 1 in which the clamping circuit includes at least one diode.
3. The double switched track-and-hold circuit of claim 1 in which the input signal is a differential signal.
4. The double switched track-and-hold circuit of claim 1 in which the switched amplifier circuits each include an emitter follower amplifier circuit.
5. The double switched track-and-hold circuit of claim 1 further including an input amplifier stage.
6. The double switched track-and-hold circuit of claim 5 in which the input amplifier stage includes an emitter follower circuit.
7. The double switched track-and-hold circuit of claim 5 in which the input buffer further includes a second amplifier stage for amplifying the input signal responsive to the input amplifier circuit.
8. The double switched track-and-hold circuit of claim 7 in which the second amplifier stage includes a differential amplifier circuit.
9. The double switched track-and-hold circuit of claim 7 in which the first switched amplifier circuit is responsive to the second amplifier stage.
10. The double switched track-and-hold circuit of claim 1 further including a current source for sinking current through the first switched amplifier circuit in the track mode and sinking current through the clamping circuit during the hold mode.
11. The double switched track-and-hold circuit of claim 1 further including a switched current source for sinking current from the first switched amplifier circuit during the hold phase to turn off the first switched amplifier circuit.