GB2313726A

GB2313726A - Voltage follower with improved power supply rejection

Info

Publication number: GB2313726A
Application number: GB9611519A
Authority: GB
Inventors: Andreas Rusznyak
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1996-06-01
Filing date: 1996-06-01
Publication date: 1997-12-03
Anticipated expiration: 2016-06-01
Also published as: GB2313726B; GB9611519D0

Abstract

A voltage follower circuit (50) for generating an output voltage signal (Vout) in dependence on an input voltage signal (Vin), comprises a first transistor (52) having a control electrode coupled to receive the input voltage signal, a load transistor (70) having a control electrode and a first current electrode coupled to the second current electrode of the first transistor (52) and to an output (61) for providing the output voltage signal, and a second transistor (54) of opposite conductivity type to the first transistor (52). The control electrode of the second transistor (54) is coupled to the second current electrode of the first transistor (52), and its first current electrode is coupled to the first current electrode of the first transistor (52). A current source (58) is coupled to the first current electrodes of the first (52) and second (54) transistors for supplying a current (Is) which is greater than the current (I) through the first transistor (52). An excess current (Id) supplied by the current source which does not flow through the first transistor (52) flows through the second transistor (54).

Description

VOLTAGE FOLLOWER CIRCUIT Field of the Invention This invention relates to voltage follower circuits, which circuits generate an output voltage signal in dependence on an input voltage signal.

Background of the Invention Voltage follower circuits which have high input impedance and which generate a voltage on a low impedance load are often used in electronic circuits. Typically such circuits comprise an active input device connected between a first supply voltage and a first terminal of a load with another terminal of the load being coupled to a second supply voltage. The load may be a resistor or a bipolar or MOS transistor used in diode configuration. The voltage generated on a common node between the active device and load depends on the voltage applied to an input electrode of the active device. If bipolar transistors are used, such follower circuits are named emitter followers. If MOS transistors are used, they are named source followers.

The following description may be applied to both kinds of follower circuits.

Voltage follower circuits may be advantageously used, for example, in phase locked loops (PLL) which comprise a voltage controlled oscillator (VCO) whose frequency is controlled by the signal delivered by the PLL loop filter. In order to ensure frequency stability of the PLL against variations in the supply voltage, it is important that the output voltage of the follower is controlled only by its input voltage and only changes very slightly with variations in supply voltage. In other words it is important that the voltage follower circuit has good power supply rejection. Such a requirement may also exist for other applications.

However, in view of the transconductance of bipolar and MOS transistors, the output voltage of the above described voltage follower circuits will depend on the supply voltage. Hence the known bipolar and MOS follower circuits have poor power supply rejection and therefore limit the performance of the circuits of which they are part.

It is therefore desirable to provide a voltage follower circuit that has improved power supply rejection over that of the above described circuits.

Summary of the Invention In accordance with the present invention there is provided a voltage follower circuit for generating an output voltage signal in dependence on an input voltage signal, the voltage follower circuit comprising a first transistor having first and second current electrodes and a control electrode coupled to receive the input voltage signal; a load transistor having a control electrode and a first current electrode coupled to the second current electrode of the first transistor and to an output for providing the output voltage signal, the load transistor further having a second current electrode coupled to a first supply voltage; a second transistor being of opposite conductivity type to the first transistor and having a control electrode coupled to the second current electrode of the first transistor, a first current electrode coupled to the first current electrode of the first transistor and a second current electrode coupled to the first supply voltage; and a current source coupled to the first current electrodes of the first and second transistors for supplying a current which is greater than the current through the first transistor, wherein an excess current supplied by the current source which does not flow through the first transistor flows through the second transistor.

An advantage of the voltage follower circuit in accordance with the invention is that it has improved power supply rejection. The current source ensures that the current through the first transistor is always the same and any excess current produced by for example variations in supply voltage, is sunk by the second transistor thereby ensuring the output voltage signal is substantially independent of such supply voltage variations.

Brief Description of the Drawings A voltage follower circuit in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIG. 1 is a schematic circuit diagram of a known voltage follower circuit; and FIG. 2 is a schematic circuit diagram of a voltage follower circuit in accordance with the present invention.

Detailed Description of the Drawings FIG. 1 shows a known voltage follower circuit 2 comprising a MOS source follower arrangement. The voltage follower circuit 2 comprises first 4, and second 6 N-channel transistors. The second transistor 6 has a source electrode coupled to a first supply voltage Vss (or ground). The first transistor 4 has a gate electrode coupled to receive an input voltage signal Vin, and a drain electrode coupled to a second supply voltage VDD. The control electrode of the second transistor 6 is coupled to its drain electrode and to the source electrode of the first transistor 4 and to an output 10 of the voltage follower circuit 2.- The voltage follower circuit 2 operates as follows.

The first transistor 4 sources a current I into the second transistor 6.

For a MOS transistor in saturation, it is well known that the drain current ID is given in first approximation by: ID =K(VGs-VT)2 (1) where: K is a scale factor determined by particulars such as the geometry of the MOS transistor, VGS is the gate to source voltage, VT is the threshold voltage of the transistor.

Using equation (1) and assuming that the first 4, and second 6 transistors have the same threshold voltage VT, the output voltage signal Vout at output 10 is: Vin*r+V(1-r) Vout = 1+ r (2) where

K4 and K6 being the scale factors of transistors 4 and 6 respectively.

From equation (2) it is therefore readily apparent that the output voltage signal Vout is dependent on the input voltage signal Vin only. Thus, when the voltage follower circuit 2 is incorporated as part the front end of a PLL so as to control the VCO, the frequency generated by the VCO will depend exclusively on the value of the input voltage signal Vin.

For applications where the frequency generated by the PLL has to be very stable, secondary effects of the voltage follower circuit 2 limit achievable performance. One parameter which significantly limits the performance is the output conductance gds of the MOS transistors which has not been accounted for in the above analysis.

The output conductance gds of a MOS transistor can be represented by a conductance through which a current flows in parallel to the drain to source current of the transistor. The value of this current depends on the drain to source voltage of the transistor.

In the voltage follower circuit 2, as the supply voltage VDD varies the conductance gds of the first transistor 4 causes an additional current to flow into the second transistor 6. The conductance of the second transistor 6 leads to a current flowing parallel to the drain current of the second transistor 6. The flow of this additional current produces a change in the output voltage signal Vout at output 10. The value of the additional current depends on the extent of the variations in VDD.

Thus, as long as the supply voltage VDD of the circuit 2 remains stable, the output voltage signal at output 10 depends on the input voltage signal Vin only. However, variations in the supply voltage VDD produce variations in the output voltage signal Vout too. These variations will be sensed by the circuit coupled to the voltage follower output at output 10.

In the case of the PLL, such variations will cause the frequency of the VCO to change and this change will be corrected by the PLL itself. In the event of a ripple on the supply voltage VDD, the resulting continuous correction by the PLL will generate phase jitter and will therefore limit the PLL's performance. The PLL's performance can be improved by improving the power supply rejection figure of the voltage follower circuit.

Referring now to FIG. 2, a voltage follower circuit 50 in accordance with a preferred embodiment of the present invention provides an output voltage signal Vout in dependence on an input voltage signal Vin but has improved power supply rejection as compared to the prior art circuit shown in FIG. 1.

The voltage follower circuit 50 comprises a first transistor 52 having a control electrode coupled to receive the input voltage signal Vin, and a load transistor 70 having a control electrode and a first current electrode coupled to the second current electrode of the first transistor 52 and to an output 61 of the voltage follower circuit 50 for providing the output voltage signal Vout.

The load transistor 70 further has a second current electrode coupled to a first supply voltage Vss (or ground). The voltage follower circuit 50 further comprises a second transistor 54 being of opposite conductivity type to the first transistor 52 and having a control electrode coupled to the second current electrode of the first transistor 52, a first current electrode coupled to the first current electrode of the first transistor 52 and a second current electrode coupled to the first supply voltage Vss. A current source 58 is coupled to the first current electrodes of the first 52 and second 54 transistors at node 60 and to a second supply voltage VDD. The current source 58 supplies a current Is which current is greater than the current I through the first transistor 52. The first transistor 52 thus cannot sink all the current delivered by the current source 58. The excess current Id supplied by the current source 58 which does not flow through the first transistor 52 flows through the second transistor 54, where Id = Is - I.

The voltage follower circuit 50 generates the output voltage signal Vout in dependence on the input voltage signal Vin substantially as described above with reference to FIG. 1. However, unlike the voltage follower circuit 2 of FIG. 1, the voltage follower circuit 50 in accordance with the present invention ensures that the output voltage signal Vout becomes substantially stable with respect to variations in the second supply voltage VDD.

If the second supply voltage VDD varies and if the current source 58 is not ideal, a conductance in parallel to this current source 58 produces an increase in the voltage signal at node 60. This renders the second transistor 54 more conductive so that the second transistor 54 sinks more current. The sinking by the second transistor 54 of the major part of the excess current caused by variations in VDD results in an output voltage signal Vout at output 61 which is substantially stable with respect to variations in VDD.

Thus, the voltage follower circuit 50 in accordance with the present invention ensures improved power supply rejection.

The current source 58 preferably comprises a current mirror 62 energised by the second supply voltage VDD, and having an output supplying a current Is to the node 60. A current supply circuit 59 supplies a current Io to an input 63 of the current mirror 62. The current mirror 62 mirrors the current Io and the mirror ratio is chosen so that the current Is at the current mirror's output is greater than the current I flowing through the first transistor 52.

Preferably, the second transistor 54 has a further body electrode 55 coupled to its first current electrode (see dotted lines in FIG. 2). Having such a body electrode coupled to the first current electrode means that the threshold voltage variation of the second transistor 54 can be cancelled by way of the well known body effect. The result is a voltage follower circuit having improved performance.

In a preferred embodiment of the present invention, the current supply circuit 59 comprises third 64 and fourth 66 transistors. The third transistor 64 has a control electrode coupled to receive the input voltage signal Vin, and a first current electrode coupled to the input 63 ofthe current mirror 62. The fourth transistor 66 has a first current electrode coupled to the second current electrode of the third transistor 64, a control electrode coupled to its first current electrode and a second current electrode coupled to the first supply voltage Vss (or ground).

The third 64 and fourth 66 transistors may be arranged so that they match the first 52 and load 70 transistors and the mirroring ratio of the second current mirror 62 is arranged to be greater than unity. This ensures that the current signal Is supplied by the current mirror 62 at its output is greater than the current I sunk by the first 52.

The current source 58 may, in addition or as an alternative to having matching transistors, further comprise a control transistor 68 for controlling the current signal Is supplied at the output of the current mirror 62 such that the supplied current signal Is is greater than the current signal I through the first transistor 52. The control transistor 68 has a control electrode coupled to the control electrode of the load transistor 70, a first current electrode coupled to the input 63 of the current mirror 62 and a second current electrode coupled to the first supply voltage Vss. If the third 64 and fourth 66 transistors do not match exactly such that the current through the third 64 and fourth 66 transistors is less than the current signal I, the additional current sunk by the control transistor 68 ensures that the current Io at the input of the second current mirror 62 is at the appropriate value to obtain Is > I.

In an alternative embodiment, the control transistor 68 is arranged to match load transistor 70 and the mirror ratio of the current mirror 62 is chosen to be greater than unity. In this arrangement, the third 64 and fourth 66 transistors may supply a current Io of low value to ensure start-up of the voltage follower circuit 50.

Alternatively, start-up may be achieved by other means in which case third 64 and fourth 66 transistors can be omitted. For example, the current source 58 may comprise the control transistor 68 and the current supply circuit 59 may comprise only one transistor, the third transistor 64 as described above. In such an arrangement, the control transistor 68 ensures that the current signal Is at the output of the second current mirror 62 is at the required level and the third transistor 64 ensures start-up of the voltage follower circuit 50. Start-up may alternatively be achieved using a large resistor coupled to the input node 63 of the current mirror 62.

In the preferred embodiment shown in FIG. 2, the first 52, third 64, fourth 66, load 70, and control 68 transistors are N-channel transistors and the second transistor 54 is a P-channel transistor. The current mirror 62 is also formed of two P-channel transistors in known manner. It will however be appreciated that it is not intended to limit the invention to this particular combination of transistors. For example, the invention may also be applied to bipolar transistors.

Claims

1. A voltage follower circuit for generating an output voltage signal in dependence on an input voltage signal, the voltage follower circuit comprising: a first transistor having first and second current electrodes and a control electrode coupled to-receive the input voltage signal; a load transistor having a control electrode and a first current electrode coupled to the second current electrode of the first transistor and to an output for providing the output voltage signal, the load transistor further having a second current electrode coupled to a first supply voltage; a second transistor being of opposite conductivity type to the first transistor and having a control electrode coupled to the second current electrode of the first transistor, a first current electrode coupled to the first current electrode of the first transistor and a second current electrode coupled to the first supply voltage; and a current source coupled to the first current electrodes of the first and second transistors for supplying a current which is greater than the current through the first transistor, wherein an excess current supplied by the current source which does not flow through the first transistor flows through the second transistor.

2. A voltage follower circuit according to claim 1 wherein the current source comprises: a current mirror having an output coupled to the first current electrodes of the first and second transistors and an input; and a current supply circuit for supplying a current to the input of the current mirror, wherein the current mirror uses the current at its input to provide a current at its output which current is greater than the current through the first transistor.

3. A voltage follower circuit according to claim 2 wherein the current source further comprises: a control transistor having a control electrode coupled to the control electrode and first current electrode of the load transistor and a first current electrode coupled to the input of the current mirror, the control transistor being arranged to control the current at the input of the current mirror such that the current supplied by the current mirror is greater than the current through the first transistor.

4. A voltage follower circuit according to claim 3 wherein the current supply circuit comprises: a third transistor having a control electrode coupled to receive the input voltage signal, a first current electrode coupled to the input of the current mirror.

5. A voltage follower circuit according to claim 2 or 3 wherein the current supply circuit comprises: a third transistor having a control electrode coupled to receive the input voltage signal, a first current electrode coupled to the input of the current mirror and a second current electrode; and a fourth transistor having a first current electrode coupled to the second current electrode of the third transistor and a control electrode coupled to its first current electrode.

6. A voltage follower circuit according to claim 5 wherein the third and fourth transistors are arranged to substantially match the first and load transistors.

7. A voltage follower circuit according to any preceding claim wherein the second transistor further comprises a body electrode coupled to its first current electrode.

8. A voltage follower circuit substantially as hereinbefore described with reference to FIG. 2 of the accompanying drawings.