Variable transconductance amplifier

A variable transconductance amplifier comprises transistors and whose emitters are connected via resistors and to a constant current source to form a differential or long tail pair. A feedback amplifier whose transconductance is controllable has inputs connected to the collectors of the transistors and outputs connected to the emitters thereof. The feedback amplifier thus supplies a differential current to the emitters of the transistors which corresponds to the product of the differential output signal of the transistors and the variable transconductance of the feedback amplifier.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a variable transconductance amplifier.
 Such an amplifier may be used, for example, in radio frequency circuitry
 as part of an automatic gain control (AGC) circuit. For example, such an
 amplifier may be used within a mixer or front end low noise amplifier
 (LNA).
 2. Description of the Prior Art
 FIG. 1 illustrates a known type of voltage controlled variable gain
 amplifier comprising NPN bipolar transistors. The amplifier comprises a
 transconductance stage in the form of transistors T1 and 12, resistors R1
 and R2 and a constant current source CC1, one of whose terminals is
 connected to ground gnd. The bases of the transistors T1 and T2 are
 connected to differential signal input terminals IN+ and IN-,
 respectively.
 The collectors of the transistors T1 and T2 are connected to a current
 steering circuit comprising transistors 1 to 4. The emitters of the
 transistors 1 and 2 are connected to the collector of the transistor T1
 whereas the emitters of the transistors 3 and 4 are connected to the
 collector of the transistor T2. The collectors of the transistors 2 and 4
 are connected to a positive supply input or line vcc. The collectors of
 the transistors 1 and 3 are connected to differential output terminals
 OUT- and OUT+ and via load resistors 5 and 6, respectively, to the supply
 line vcc. The bases of the transistors 1 and 3 are connected to a first
 gain control input "agc adjust+" whereas the bases of the transistors 2
 and 4 are connected to a second gain control input "agc adjust". In use, a
 differential gain control voltage is supplied to the gain control
 terminals so that, when maximum gain is required, all of the signal
 current from each of the transistors T1 and T2 is steered through the
 respective load resistor 5 and 6. Conversely, when minimum gain is
 required, the signal current is steered via the transistors 2 and 4 to the
 supply line vcc.
 Although a voltage controlled variable gain amplifier of the type shown in
 FIG. 1 may be used in an AGC arrangement of a radio frequency (RF) tuner,
 it has certain disadvantages when used in such an application. For
 example, as the gain of the amplifier decreases, the noise figure (NF)
 increase. Also, using NPN technology (or NMOS technology for field effect
 transistors) increases the difficulty of connecting the outputs to, for
 example, a mixer of a frequency changer.
 Further, in order to handle large amplitude input signals, the
 transconductance input stage requires sufficient power to maintain
 linearity. This may be achieved by providing the resistors R1 and R2 with
 relatively high values and using a low quiescent current, in which case
 the noise figure of the transconductance stage is compromised.
 Alternatively, higher quiescent current and lower values of the resistors
 R1 and R2 may be used but this results in increased power dissipation.
 SUMMARY OF THE INVENTION
 According to the invention, there is provided a variable transconductance
 amplifier comprising first and second amplifying devices whose common
 terminals are connected via first and second resistors, respectively, to a
 first current source, and a feedback amplifier of variable
 transconductance having outputs connected to the common terminals of the
 first and second devices for supplying a differential current
 corresponding to the product of a differential output signal of the first
 and second devices and the variable transconductance.
 The first current source may comprise a first constant current source.
 The first and second devices may be of a first conduction type and the
 feedback amplifier may comprise amplifying devices of a second conduction
 type opposite the first type.
 The feedback amplifier may have an input stage comprising third and fourth
 devices whose control terminals are connected to output terminals of the
 first and second devices, respectively, and whose common terminals are
 connected via third and fourth resistors, respectively, to a second
 current source.
 The second current source may comprise a second constant current source.
 The output terminal of the third device may be connected to the common
 terminals of fifth and sixth devices whose output terminals are connected
 to the common terminal of the first device and a first power supply input,
 respectively, the output terminal of the fourth device may be connected to
 the common terminals of seventh and eighth devices whose output terminals
 are connected to the common terminal of the second device and the first
 power supply input, respectively, the control terminals of the sixth and
 eighth devices may be connected to a first gain control input and the
 control terminals of the fifth and seventh devices may be connected to a
 second gain control input.
 The output terminals of the first and second devices may be connected via
 fifth and sixth resistors to the common terminals of ninth and tenth
 devices, respectively, whose, control terminals are arranged to receive a
 substantially constant bias voltage.
 The output terminals of the first and second devices may be connected via
 seventh and eighth resistors to first and second signal inputs,
 respectively, of a mixer comprising: eleventh and twelfth devices, whose
 common terminals are connected to the first signal input, whose control
 terminals are connected to first and second local oscillator inputs and
 whose output terminals are connected to first and second outputs,
 respectively; and thirteenth and fourteenth devices whose common terminals
 are connected to the second signal input, whose control terminals are
 connected to the first and second local oscillator inputs and whose output
 terminals are connected to the first and second outputs, respectively.
 Each of the devices may comprise a transistor. Each of the devices may
 comprise a bipolar transistor and the common, control and output terminals
 may comprise emitter, base and collector electrodes, respectively. Each of
 the first and second devices may comprise an NPN transistor.
 The common, control and output terminals of an amplifying device refer to
 the normal use of such a device in an amplifying and inverting
 configuration. Thus, as mentioned hereinbefore, in the case of bipolar
 transistors the common, control and output terminals are the emitter, base
 and collector electrodes. In the case of field effect transistors, the
 common, control and output terminals are the source, gate and drain
 electrodes, respectively.
 It is thus possible to provide a low noise variable transconductance
 amplifier whose pain may be varied without unacceptably comprising the
 noise figure. Also, the use of complementary technology means that large
 supply voltages are not required. This arrangement also facilitates the
 use of the amplifier as part of a mixer, such as a Gilbert mixer, which
 may use the amplifier as its input stage and at the same time provide
 automatic gain control.

Like reference numerals refer to like parts throughout the drawings.
 DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The variable transconductance amplifier illustrated in FIG. 2 comprises an
 input stage in the form of a differential or long tail pair. In
 particular, the input stage comprises NPN transistors T1 and T2 whose
 gates are connected to differential input terminals IN+ and IN-. The
 emitters of the transistors T1 and T2 are connected via resistors R1 and
 R2, respectively, to one terminal of a constant current source CC1 whose
 other terminal is connected to ground gnd. The collectors of the
 transistors T1 and T2 are connected to differential output terminals OUT-
 and OUT+. The collectors of the transistors T1 and T2 are also connected
 to a feedback amplifier having variable transconductance and comprising a
 transconductance stage 7 and a variable gain stage 8. The stage 8 has
 differential outputs which are connected to the emitters of the
 transistors T1 and T2.
 In use, the stage 7 converts the differential output signal of the
 amplifier into a current which is varied by the stage 8 in accordance with
 the desired amplifier gain and fed back to the emitters of the transistors
 T1 and T2. In particular, by varying the gain of the stage 8, the
 transconductance of the "main" amplifier comprising the transistors T1 and
 T2 can be controlled. Such an arrangement may be used as part of an AGC
 circuit and provides variable gain without substantially comprising the
 noise figure. Further, it is not necessary to make any substantial
 compromise between noise figure and power dissipation of the amplifier.
 As shown in FIG. 3, the stages 7 and 8 are based on PNP transistors T3 to
 T8. The transistors T3 and T4 comprise another long tail pair whose bases
 are connected to the collectors of the transistors T1 and T2,
 respectively, and whose emitters are connected via resistors R3 and R4,
 respectively, to one terminal of another constant current source CC2,
 whose other terminal is connected to a positive supply input or line vcc.
 The collector of the transistor is connected to the emitters of
 transistors T5 and T6 whereas the collector of the transistor T4 is
 connected to the emitters of transistors T7 and T8. The collectors of the
 transistors T5 and T7 are connected to the emitters of transistors T1 and
 T2, respectively, whereas the collectors of the transistors T6 and T8 are
 connected to ground gnd. The bases of the transistors T6 and T8 are
 connected to a first gain control input "agc adjust+" whereas the bases of
 the transistors T5 and T7 are connected to a second gain control input
 "agc adjust-".
 The amplifier shown in FIG. 3 is provided with a casecode output
 arrangement comprising NPN transistors T9 and T10. The emitters of the
 transistors T9 and T10 are connected to the collectors of the transistors
 T1 and T2 via resistors R5 and R6, respectively. The bases of the
 transistors T9 and T10 are connected to a bias input for receiving a
 substantially constant voltage bias (which input may be made available for
 providing adjustment of the bias voltage). The collectors of the
 transistors T9 and T10 are connected to "open-collector" differential
 outputs OUT- and OUT+.
 In use, a differential input signal is supplied to the differential inputs
 IN+ and N-. A differential gain control voltage is supplied to the gain
 control inputs so as to control the gain of the amplifier The transistors
 T3 to T8 constitute a variable transconductance amplifier which feeds back
 to the emitter of the transistors T1 and T2 a current which is
 substantially proportional to the product of the amplifier differential
 output signal and the variable transconductance as determined by the
 control voltage. For maximum gain, the control voltage is such that little
 or no current flows through the transistors T5 and T7. In order to reduce
 the gain or transconductance of the amplifier, the control voltage is
 varied so as to increase the current through the transistors T5 and T7.
 This has the effect of "boosting" the values of the resistors R1 and R2 so
 as to increase the local feedback and hence reduce the transconductance of
 the transistors T1 and T2. However, this does not substantially affect the
 noise figure of the amplifier.
 By fixing the ratio of the value of each of the resistors R1 and R2 to the
 internal emitter resistance of each of the transistors T1 and T2, the
 distortion performance of the amplifier is substantially fixed under
 maximum gain conditions. However, at minimum gain, the distortion is
 reduced because the value of each of the resistors R1 and R2 is actively
 boosted while maintaining the internal emitter resistance of the
 transistors T1 and T2 substantially constant. In order to achieve this,
 the output current of the constant current source CC2 must be smaller than
 that of the constant current source CC1.
 The use of complementary transistor technology means that large supply
 voltages are not required.
 Although the amplifier is shown as being embodied by bipolar transistors,
 such an amplifier may be embodied using other active amplifying devices,
 such as field effect transistors of junction or insulated gate type.
 The variable transconductance amplifier may be used as part of a mixer and
 FIG. 4 illustrates such an arrangement with a Gilbert mixer "stacked" on
 the amplifier. The amplifier differs from that shown in FIG. 3 in that the
 resistors R5 and R6 and the transistors T9 and T10 are omitted and
 replaced by resistors R7 and R8 and NPN transistors T11 to 14. The
 emitters of the transistors T11 and T12 are connected via the resistor R7
 to the collector of the transistor T1 whereas the emitters of the
 transistors T13 and T14 are connected via the resistor 8 to the collector
 of the transistor T2. The collectors of the transistors T11 and T14 are
 connected to a first differential output OUT+ whereas the collectors of
 the transistors T12 and T13 are connected to a second differential output
 OUT-. The bases of the transistors T11 and T13 are connected to a first
 differential local oscillator input LO+ whereas the bases of the
 transistors T12 and T14 are connected to a second differential local
 oscillator input LO-.
 The gain control voltage for the gain control inputs is supplied from a
 suitable circuit of a tuner in any suitable (e.g. known) way so as to vary
 the gain of the variable transconductance amplifier. Automatic gain
 control and frequency changing can therefore be performed in a single
 circuit arrangement or stage. The AGC function is provided with a lower
 noise figure than for known arrangements.