GB2280073A

GB2280073A - Amplifier gain control circuit

Info

Publication number: GB2280073A
Application number: GB9313460A
Authority: GB
Inventors: Edward John Wemyss Whittaker
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1993-06-30
Filing date: 1993-06-30
Publication date: 1995-01-18
Anticipated expiration: 2013-06-30
Also published as: GB2280073B; GB9313460D0

Abstract

A linear amplifier includes first and second gain cells (11, 12) each comprising a long-tailed transistor pair arrangement. A control circuit 13 supplies each gain cell with a control current whose magnitude is proportional to the absolute temperature so as to provide temperature independent transconductance for each long-tailed pair. This provides a substantially linear relationship between an input control signal and the amplifier gain. Transistor circuits are described for the long-tailed transistor pair arrangement (Fig. 2) and the control circuit (Fig. 4). <IMAGE>

Description

AMPLIFIER CIRCUIT This invention relates to integrated circuit amplifiers, and in particular to amplifiers having a controllable gain.

Linear amplifiers, i.e. amplifiers whose gain varies in a generally linear manner in response to an input control voltage or current, are in various control systems and in consumer product applications e.g. as automatic gain control elements. The gain of such an amplifier is required to change by a predetermined number of decibels e.g. for each applied volt of the control input. A particular problem with currently available devices is their significant departure from linearity over the control voltage or current range.

This has limited their use in critical applications.

The object of the invention is to minimise or overcome this disadvantage.

According to the invention there is provided an amplifier circuit including one or more long tailed transistor pair gain elements, and means for providing to each said long tailed pair a control current proportional to the absolute temperature whereby to provide a temperature independent transconductance of each said long tailed pair.

According to the invention there is further provided a linear amplifier circuit having a gain proportional to an input control signal, the circuit including first and second long tailed transistor pair gain elements, and a control current source adapted to provide to each said long tailed pair a respective control current proportional to the absolute temperature, wherein the control current source comprises a linear transconductance stage incorporating a further long-tailed transistor pair whose tail current is determined by a temperature independent current source, a multiplier stage whose gain is determined by the ratio of a current proportional to the absolute temperature and the current provided by said temperature independent current source, and output means for deriving said respective control current from the output of the multiplier.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 shows a block schematic diagram of the amplifier; Fig. 2 shows a gain cell element for use in the amplifier of Fig. 1; Fig. 3 illustrates the derivation of a temperature dependent current in the amplifier of Fig. 1; and Fig. 4 shows a control circuit for the amplifier.

Referring to Fig. 1, this shows a configuration of the linear amplifier and its control circuit incorporating two gain cells. In principle, any number of gain cells may be cascaded multiplying the gain of the amplifier (in dBs) but the number of stages employed. As shown in Fig. 1 each gain cell 11, 12 comprises a two input/two output differential amplifier. Typically one input to the gain cells comprises a reference voltage. The gain of each cell 11, 12 is determined by a control circuit 13 which supplies each gain cell with a control current IA, IB in response to a control signal applied to the input of the control circuit.

Fig. 2 shows the construction of a gain cell element which comprises three long tailed transistor pairs each having a transconductance proportional to tail current and inversely proportional to absolute temperature. The inner pair Q7 and Q8 have a gain maximum centred around zero input volts whereas the two outer pairs Q2 and Q9, and Q5 and Q11 have gain maxima at -KT(Loge10)/q and KT(Loge 10 )/q (approx). -60mV and +60mV at room temperature) respectively. These three outputs are summed in R4 and R5 and by the appropriate ratioing of tail currents through Q1, Q4 and Q6 by choice of emitter area and resistor values R1, R2 and R3, then a low distortion transconductance stage results capable of handing signals of approx. +/-60mV (at room temperature).The resistors R1, R2 and R3 as well as improving matching also reduce noise. This circuit provides temperature independent gain if the control current is made proportional to absolute temperature (PTAT). The circuit of Fig. 2 is capable of giving wide bandwidth if the stages are operated at high current but collector base capacitance will cause significant signal feedthrough at low currents. The signals on the collectors of Q7 and Q8 are antiphase and a degree of collector base capacitance compensation is available by the use of appropriately sized devices Q3 and Q10 which feed inphase signals back to the bases of the input transistors. For wide bandwidth combined with a large range of gain control it is necessary to restrict the variation in current in each gain cell and use a cascade of cells to obtain the range of gain variation.

In Fig. 3, the use of a bandgap reference voltage (approx. 1.22v) provides a current into the transistor Q1. This current is proportional to absolute temperature because of the variation of the Vbe with temperature. Transistor Q2 mirrors this current to provide an output.

This technique is used as the basis of the control circuit shown in Fig. 4. This circuit comprises a multiplier stage (transistor Q2, Q8, Q9 and Q10), a linear transconductance stage (transistors Q1, Q3, Q4, Q5, Q6 and Q7) and an output follower stage. In the circuit of Fig. 4, transistor Q11 is fed from the bandgap voltage (+VBG) via R6. With zero volts on the control input transistors Q9 and Q10 run at the same current and the collectors of Q9 and Q10 are set at the same potential as transistor Q11 by the Proportional to Absolute Temperature (PTAT) current source 14 described above with reference to Fig. 3. The resistors R5 and R7 are equal in value and the resistors R3 and R4 are also equal.Transistor Q11 sets up a low impedance source for resistor R5 and so, with operational amplifier OPAMP1, a differential stage is formed which is less sensitive to variations of the current 14 with respect to the current flowing in transistor Q11. The opamp thus maintains the bases of transistors Q12 and Q13 at the same potential as the base and collector junctions of transistor Q11.

In order to provide the exponential gain variation with control voltage the circuit of Fig. 4 uses the exponential characteristic of the base emitter junction: l=l0(exp(qVe/l < T)-1) - - - (1 ) (where 1o is the saturation current, q is the charge on the electron, K is Boltzmann's constant, T is absolute temperature and Vbe is the base emitter voltage).

For realistic operating currents the -1 term at the end of the equation (1) may be ignored and it can then be seen that if the base voltage of transistors Q12 and Q13 can be offset from that of transistor Q11 by an amount proportional to the control input and proportional to absolute temperature.

AVbeooVcontroiT (VControl is the control input voltage) The ratio of currents in transistors Q12 and Q13 to that in Q11 is thus exponentially related to the control voltage and temperature independent.

Considering the case when there is a control input voltage with respect to ground: transistors Q1, Q3, Q4, Q5, Q6 and Q7 form a linear transconductance stage with a stage gain set by: gm=1/(R1 +R2) The stage is coupled to the negative power supply ... via a constant current source 12 which is substantially temperature independent.

Transistor Q2, Q8, Q9 and Q10 form a Gilbert multiplier with a current gain set by: gain=14/12 Since 14 is arranged to be proportional to absolute temperature the current imbalance, set up by the control input, between the collectors of transistors Q9 and Q10 also is proportional to absolute temperature.

Thus the voltage on the bases of transistors Q12 and Q13 follow the voltage on Q11 base but are reduced by an amount proportional to the control input and absolute temperature VQ12, 13 =VQ11 -K(Vcontroi) where K is a factor set by (14/(12(R1+R2))*R4R5/(R4sR5).

This provides a high degree of linearity of the gain of the gain cells controlled by the respective output currents of the transistors Q12 and Q13.

The linear amplifier circuit described above is of particular application to signal processing in radar range finding equipment where a high degree of linearity is essential to provide accuracy. It will however be appreciated that the technique is also of more general application.

Claims

CLAIM:

1. An amplifier circuit including one or more long tailed transistor pair gain elements, and means for providing to each said long tailed pair a control current proportional to the absolute temperature whereby to provide a temperature independent transconductance of each said long tailed pair.

2. A linear amplifier circuit having a gain proportional to an input control signal, the circuit including first and second long tailed transistor pair gain elements, and a control current source adapted to provide to each said long tailed pair a respective control current proportional to the absolute temperature, wherein the control current source comprises a linear transconductance stage incorporating a further long-tailed transistor pair whose tail current is determined by a temperature independent current source, a multiplier stage whose gain is determined by the ratio of a current proportional to the absolute temperature and the current provided by said temperature independent current source, and output means for deriving said respective control current from the output of the multiplier.

3. A circuit as claimed in claim 2, wherein said current proportional to the absolute temperature is provided by a current source comprising a bipolar transistor driven from a bandgap reference voltage supply.

4. A circuit as claimed in claim 2 or 3. wherein each said gain cell comprises a multi-transistor long-tailed pair arrangement, two paired transistors of the arrangement having a gain maximum centered around a zero voltage input and respective further paired transistors having a gain maximum centered around positive and negative voltage inputs.

5. A linear amplifier circuit substantially as described herein with reference to and as shown in the accompanying drawings.

6. A radar range finding processor incorporating one or more linear amplifiers as claimed in any one of claims 1 to 5.