Fast high side switch for hard disk drive preamplifiers

1. A fast high side switch, for fast switching, from a high impedance nonconductive state to a low impedance conductive state, between two terminals, the switch comprising:

a control circuit for turning the switch on and off at a fast speed, said control circuit including a first input terminal and a second input terminal;

a pnp transistor coupled to the control circuit, said pnp transistor being turned on when the first input terminal is high;

a first npn transistor, the base of said first npn transistor coupled to the collector of said pnp transistor, and said first npn transistor going into saturation when said pnp transistor turns on;

a second npn transistor, the collector of said second npn transistor coupled to a first of the two terminals, the emitter of said second npn transistor coupled to a second of the two terminals, and the base of said second npn transistor coupled to the emitter of said first npn transistor,

wherein said second npn transistor turns on after said first npn transistor goes into saturation, thereby closing the switch and providing a connection between the two terminals; and

a capacitor, connected between the base and the emitter of said second npn transistor,

wherein the emitter current of said first npn transistor is divided between the base current of said second npn transistor and said capacitor.

2. The fast high side switch of claim 1, further comprising a clamp circuit connected to the base of the first npn transistor.

3. The fast high side switch of claim 2, wherein said clamp circuit comprises a resistor and a plurality of transistors.

4. The fast high side switch of claim 1, wherein said pnp transistor is turned off when the second input terminal is high and the first input terminal is low.

5. The fast high side switch of claim 4, wherein said second npn transistor stays high until said capacitor is discharged and then turns off.

6. The fast high side switch of claim 1, said control circuit further comprising a differential npn transistor pair, wherein the tail current of the differential pair may be independently controlled by a third input terminal.

7. The fast high side switch of claim 6, wherein the third input terminal can further be used to turn off the fast high side switch irrespective of the first input terminal and the second input terminal.