GB2298531A

GB2298531A - Voltage follower circuit providing filtered output voltage

Info

Publication number: GB2298531A
Application number: GB9503450A
Authority: GB
Inventors: Christopher Eric Brice; Stephen Edward Cooper
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1995-02-22
Filing date: 1995-02-22
Publication date: 1996-09-04
Also published as: GB9503450D0

Abstract

A voltage follower circuit for smoothing an output voltage from an unregulated source and providing a filtered output voltage includes a filtered reference voltage circuit 12, a closed control loop 14 and a MOSFET active filter 20. A first reference voltage 19 is provided by diode 46, network Z1, Z2 and a capacitor C2. A second reference voltage 18 is obtained from a feedback network 16 connected to the output 15. A feedback amplifier 22 controls the MOSFET 20.

Description

VOLTAGE FOLLOWER Field of the Invention This invention relates in general to regulating or smoothing an output from unregulated sources, and more particularly to a voltage follower for smoothing output voltage from a power supply line.

Background to the Invention When designing amplifiers in communications systems, it is necessary to ensure that power supply lines are regulated or smoothed. The power supply lines are smoothed so that harmonics found on the raw supply lines do not cause transmission of spurious frequencies.

FIG. 1 shows an example of how power supply noise transmits undesired spurious frequencies. FIG. la shows a power supply noise spectrum. FIG. 1b shows a spectrum at the input of the an amplifier. FIG.

1c shows the spectrum at the output of the amplifier. The spurious frequencies generated distort modulation and can cause severe blocking problems to other users in transmitters, or they may desensitize or block in receivers.

One solution to this problem is to use a passive low pass filter (LPF), either inductor/capacitor (LC) or resistor/capacitor (RC). This solution however has the following disadvantages: 1. Component values are so large as not to be practically realizable (LC) if very low corner frequencies are desired.

2. Power dissipation within the resistive element is prohibitive (RC).

Furthermore, standard series and parallel regulators are relatively simple to implement but have two basic drawbacks when used on a power line.

1. They require that the input voltage be a minimum level above the output voltage.

2. If the input voltage rises the power dissipation is increased. With a standard tolerance of output voltage from a PSU over normal operating condition this will require that the pass transistors of the regulator be heat sunk due to the power dissipation.

There are a number of regulator designs that are well understood and documented that can be used as in a similar application, but have the penalties outlined above. The other alternative is the use of a bipolar active filter.

In power applications the corner frequencies of these filters are limited by the beta of the pass transistor. The alternative to increasing the beta of the pass element is to use a Darlington transistor as the control element rather a simple single stage transistor. This however has the disadvantage of increasing the minimum voltage that must be dissipated across the pass transistor and increases its power consumption. However even in this case the beta of the Darlington pass transistor pair is limited to approximately 500 in saturation, limiting the minimum realisable corner frequency of the filter.

Summary of the Invention According to the present invention, there is provided a voltage follower for outputting a filtered output voltage including a filtered reference voltage circuit having an input voltage Vi and outputting a first reference voltage Vrefl, a closed control loop where the first reference voltage Vrefl is as an input and a second reference voltage Vref2 is an input and a control loop voltage Vcl is outputted, and a FET having as inputs the input voltage Vi and the control loop voltage Vcl and outputting a filtered output voltage VO.

In a preferred embodiment the voltage follower includes a feedback circuit where the filtered output voltage VO is fed back to the closed loop control circuit as the second reference voltage Vref > .

Brief Description of the Drawing FIG. 1 shows the effect of power supply line noise on an amplifier.

FIG. 2 shows a voltage follower circuit according to the preferred embodiment of the present invention.

Detailed Description of the Preferred Embodiment FIG. 2 shows a voltage follower according to the preferred embodiment of the present invention. The voltage follower includes a filtered reference voltage circuit 12 coupled to a power supply line and having an input voltage (Vi) 10. The filtered reference circuit outputs a first reference voltage 18. The first reference voltage 18 is inputted to a closed control loop 14. A second reference voltage 19 is also coupled to the closed control loop 14 as input. The closed control loop 14 outputs a control loop voltage 17. The input voltage 10 and the control loop voltage 17 are coupled to a field effect transistor (FET) 20, preferably a MOSFET. The MOSFET 20 outputs a filtered output voltage VO 15.

The voltage follower of FIG. 2 includes a feedback circuit 16 where the filtered output voltage VO 15 is fed back to the closed loop control circuit 14 as the second reference voltage 19. A reservoir capacitor 21 is coupled to the filtered output voltage VO 15.

The filtered reference voltage 12 circuit may be implemented in a low pass filter as shown in FIG. 2 by diode (D1) 46, impedances (Z1, Z2) 41, 42 and capacitor (C2) 32. The closed control loop 14 is implemented by an operational amplifier (U1) 22 and impedance (Z3) 43. The feedback circuit 16 is implemented by impedances (Z4, Z5) 44, 45.

The voltage follower of the present invention provides a smoothed output voltage from an unregulated source, or power supply, using a MOSFET (Q1) 20 as a series control transistor 20, the operational amplifier 22 to control the output voltage VO 15 and the feedback networks 41-45 to derive the control signals input 18, 19 to the operational amplifier 22.

The voltage follower of the present invention may be also referred to as a MOSFET superfilter. With careful design the voltage drop across the MOSFET series control element 20 will always be less than the drop of a bipolar transistor.

The corner frequency of the low pass filter is set by the capacitor (C2) 32 and impedance networks( Z1, Z2) 41, 42. The active filter will be able to equalize for incoming voltage ripple providing that the incoming voltage is greater than the output voltage 15 plus the voltage drop through the MOSFET 20 when it is saturated. If the drop is momentary the voltage will be held up by the charge stored in capacitor C3. The output voltage is set by the potential division of Vi by C1, Z1, Z2, Z4 and Z5.

A preferred embodiment of the present invention, a power MOSFET superfilter, is shown in FIG. 2 includes impedances (Z1, Z2, Z3, Z4, Z5) 4145, capacitors (C1, C2, C3) 21,31,32, P channel MOSFET transistor (Q1) 21, operational amplifier (U1) 22 and diode (D1) 46. Diode D1 may be optionally included if it is desirable to maintain a fixed difference between input and output mean voltages. The impedances may be either simple resistors or reactive networks.

The feedback networks (Z1, Z2, Z4, Z5) 41-42, 44-45 help ensure that the feedback amplifier 22 does not have to exceed its common mode input range. Impedance networks Z1 and Z3 set the gain of the error amplifier and the system stability.

Capacitors (C1, C3) 31,21 act as decoupling capacitors helping to remove any spurious high frequencies. In addition capacitor (C3) 21 will also help to stabilize the loop and act as a reservoir capacitor should the input voltage instantaneously fall below a minimum input voltage.

The mode of operation is as follows: a first reference voltage 18 is a smoothed DC voltage derived at the input of the amplifier (U1) 22 by networks (Z1, Z2) 41-41, diode (D1) 46, and capacitor (C2) 32. The operational amplifier 22 will change the gate voltage of the MOSFET 20 to maintain the two inputs of the operational amplifier 22 at similar levels.

The non-inverting input 19 is derived from the potential division of the output voltage VO 15 by the feedback network (Z4, Z5) 16. If the output voltage VO 15 falls relative to input voltage Vi 10 the non inverting voltage 19 will fall relative to the inverting voltage 18 this will cause the Ql's gate voltage to drop allowing more current through Q1 20 and hence increasing the output voltage Vg. As Q1 is a linear resistive control element, if the input voltage Vi falls sufficiently the output voltage Vg will also fall.

To assure smoothing of the output voltage VO 15, the input voltage Vi 10 must be above a given level.

The error amplifiers feedback network (Z3) 43 in conjunction with the input network (Z1, Z2) 41, 42 can be used to set the stability and regulation characteristics of the system.

As the output voltage VO is always a fixed level below the input voltage (if Z1/Z2=Z3/Z4 and Z3 > > Z1) the power dissipation for a fixed current output will be relatively constant, simplifying the thermal design.

More than one series control transistor may be used for high current applications, however this will require the use of a compensating amplifier or source resistors to ensure the current though each pass transistor is approximately equal.

The practical considerations when implementing this system are to ensure that the control system is stable. The high frequency response of the serial control MOSFET will typically be limited by the ability of the operational amplifier to drive the large capacative load of the MOSFET's gate. This will be compensated for by the increased effectiveness of the capacitors at high frequencies.

A power superfilter of the present invention includes an embodiment where Z 1=Z2=Z4=Z5= 10K(ohms, Z3= 1M/ohms, C 1=C2=C3= lOOmicroF, U1=MC33171, Q1=MTP23P06.

The voltage follower of the present invention provides many advantages over the prior solutions. The preferred embodiment, MOSFET superfilter, provides an unique method of smoothing an output voltage from an unregulated power source as used in receivers in communications systems. As the smoothing action of the MOSFET superfilter is not dependent on the input voltage Vi it will continue to provide regulation as the input voltage Vi falls. This makes it particularly useful for operating using a battery powered supply.

As the MOSFET superfilter's voltage drop is below that of standard regulators it will operate more power efficiently. Thus, as the MOSFET superfilter's voltage drop is below that of standard regulator it can provide a higher output voltage than a conventional regulator.

The MOSFET superfilter can provide a rapidly varying load with a constant voltage source. The current capacity of the MOSFET superfilter can be increased by paralleling a number of superfilters with a common current compensating amplifier.

The MOSFET superfilter provides a unique implementation of an offset voltage follower.

Claims

1. A voltage follower for outputting a filtered output voltage, comprising: a filtered reference voltage circuit 12 having an input voltage Vi 10 and outputting a first reference voltage 18; a closed control loop 14 having the first reference voltage 18 as an input and a second reference voltage 19 as an input and outputting a control loop voltage 17; and a FET 20 having as inputs the input voltage Vi 10 and the control loop voltage 17 and outputting a filtered output voltage VO 15.

2. The voltage follower of claim 1 further comprising a feedback circuit 16 where the filtered output voltage VO is fed back to the closed loop control circuit as the second reference voltage 19.

3. The voltage follower of claim 1 further comprising a reservoir capacitor 21 coupled to the filtered output voltage VO.

4. A voltage follower substantially as herein described with reference to FIG. 2 of the drawing.