Low output impedance, high speed and high voltage generator for use in driving a capacitive load
A voltage generator circuit uses a feedback loop to regulate an output voltage at an output node. A pair of opposite conductivity source-follower transistors are coupled to the output node. A first one of the source-follower transistors operates to provide a fast current transient for charging a capacitive load that is switchably connected to the output node. A second one of the source-follower transistor operate under feedback control to regulate the voltage level at the output node.
1. A voltage generator circuit, comprising:
a differential amplifier having a first input receiving a reference voltage and a second input receiving a feedback voltage;
a first transistor of a first conductivity type having a gate terminal coupled to an output of the differential amplifier and having a drain terminal;
a second transistor of a second conductivity type configured as a source-follower with a gate terminal coupled to the drain terminal of the first transistor and a source terminal coupled to a voltage output node;
a third transistor of the first conductivity type configured as a source-follower with a gate terminal coupled to a bias current node and a source terminal coupled to said voltage output node; and
a bias circuit configured to generate a bias current for application to said bias current node.
2. The voltage generator circuit of claim 1 , further comprising:
a fourth transistor of the second conductivity type configured as a diode-connected device with a gate terminal and drain terminal coupled to the drain terminal of the first transistor; and
a fifth transistor of the first conductivity type configured as a diode-connected device with a gate terminal and drain terminal coupled to bias current node.
3. The voltage generator circuit of claim 2 , wherein a source terminal of the fourth transistor is coupled to a source terminal of the fifth transistor at an intermediate node.
4. The voltage generator circuit of claim 1 , further comprising an additional transistor having a gate terminal and a drain terminal coupled to the first transistor and a source terminal coupled to a ground reference.
5. The voltage generator circuit of claim 1 , wherein a drain terminal of the second transistor is coupled to a ground reference and a drain terminal of the third transistor is coupled to a supply voltage.
6. The voltage generator circuit of claim 1 , further comprising a voltage sense circuit coupled to the voltage output and configured to generate said feedback voltage as a function of a sensed voltage at the voltage output node.
7. A device, comprising:
the voltage generator circuit of claim 1 ;
a capacitive load; and
a switching circuit configured to selectively connect the voltage output to the capacitive load.
8. The device of claim 7 , wherein selective connection by the switching circuit is driven in response to a periodic control signal.
9. The device of claim 7 , further comprising:
a fourth transistor of the second conductivity type configured as a diode-connected device with a gate terminal and drain terminal coupled to the drain terminal of the first transistor; and
a fifth transistor of the first conductivity type configured as a diode-connected device with a gate terminal and drain terminal coupled to bias current node.
10. The device of claim 9 , wherein a source terminal of the fourth transistor is coupled to a source terminal of the fifth transistor at an intermediate node.
11. The device of claim 7 , further comprising an additional transistor having a gate terminal and a drain terminal coupled to the first transistor and a source terminal coupled to a ground reference.
12. The device of claim 7 , wherein a drain terminal of the second transistor is coupled to a ground reference and a drain terminal of the third transistor is coupled to a supply voltage.
13. The device of claim 7 , further comprising a voltage sense circuit coupled to the voltage output and configured to generate said feedback voltage as a function of a sensed voltage at the voltage output node.
14. A voltage generator circuit, comprising:
a capacitive output node;
a first source-follower transistor of a first conductivity type configured to source current to the capacitive output node;
a second source-follower transistor of a second conductivity type configured to sink current from the capacitive output node;
a differential amplifier having a first input receiving a reference voltage and a second input receiving a feedback voltage derived from a voltage at the capacitive output node; and
wherein said second source-follower transistor is controlled in response to an output of the differential amplifier.
15. The circuit of claim 14 , wherein the first conductivity type is n-type and the second conductivity type is p-type.
16. The circuit of claim 14 , further comprising:
a first diode-connected transistor coupled in a current mirroring configuration with the first source-follower transistor; and
a second diode-connected transistor coupled in a current mirroring configuration with the second source-follower transistor.
17. The circuit of claim 16 , wherein said first and second diode-connected transistors have source-drain paths coupled in series.
18. The circuit of claim 17 , further comprising current source circuit configured to source current to the first diode-connected transistor.
19. The circuit of claim 17 , further comprising a transistor having a source-drain path coupled in series with the source-drain path of the second diode-connected transistor, and wherein a control terminal of said transistor is coupled to the output of the differential amplifier.
20. The circuit of claim 14 , wherein the first source-follower transistor is controlled in response to a bias current.