GB2218585A - Transconductance mixer - Google Patents

Abstract

A mixing circuit comprises first and second transistors Q5, Q7 connected in differential configuration having as an emitter load a third transistor Q2, a first signal S2 being applied to the base of the first transistor Q5 and a second signal S1 being applied to the base of the third transistor Q2, whereby to provide a signal across an output load XL of the first transistor proportional to the product of the first and second signals, and a transimpedance amplifier 12 being coupled to said output load to provide a voltage output signal dependent on the current flow in said output load. <IMAGE>

Description

flRANSCONDUCTANCE MIXER This invention relates to a circuit for mixing two input signals to produce a modulated signal in which onee input signal modulates the other input signal.

A known type of mixing circuit, known as a transconductance mixer, is shown in Figure 1 in which an input signal S1 to be modulated, which may be a sine wave or any more complex signal is applied to the base of transistor Ql which forms part of a differential pair with transistor Q2 the base of which receives an antiphase version S1' of the input signal. The emitters of transistors Q1 and Q2 each contain degeneration resistors RE which are connected to a common transistor Q3.

The modulating signal S2, commonly a square wave, is applied to the bases of transistors Q4 and Qs which form differential pairs with transistors Q6 and Q7, transistor Q2 forming the load of transistor pair Q4 Q6 and transistor Q1 forming the load for transistor pair Qs Q7.

The collectors of transistors Q4 and Q7 are connected together and the collectors of transistors Q6 and Qs are connected in common to a load impedance XL. Thus in operation of the positive part of the cycle of square wave S2, Q4 and Q5 are held on so that the signal S1 appears at load impedance XL. In the negative part of the cycle of square-wave S2, Q6 and Q7 are held so that antiphase signal S1' appears at load impedance XL. This mixing effect is indicated schematically by the waveform diagrams of Figure 1 A.

The advantages of the circuit shown in Figure 1 are that of high linearity in mixing, gain and bandwidth. However, it can be shown from an analysis of the circuit of Figure 1, that the range of linearity available is directly proportional to the d.c. voltage dropped across the degeneration resistor RE. Hence for a high linearity range the voltage across this resistor must be maximised. However a significant proportion of the available supply voltage is taken up at the output load XL so that the output signal voltage does not saturate the output transistors. This problem becomes more severe as the mixer gain increases, and hence there is a trade-off between gain and linearity in this configuration.A further problem with this configuration of mixer is that the output bandwidth is limited by the time constant of the load impedance XL with the parasitic capacitance of the output transistors.

With a view to overcoming these problems, the present invention provides a mixing circuit comprising first and second transistors connected in differential configuration having as an emitter load a third transistor, a first signal being applied to the base of the first transistor and a second signal being applied to the base of the third transistor, whereby to provide a signal across an output load of the first transistor proportional to the product of the first and second signals, and a transimpedance amplifier being coupled to said output load to provide a voltage output signal dependent on the current flow in said output load.

Thus in accordance with the invention, the provision of a transimpedance amplifier as an output stage permits the mixing circuit to be designed with a relatively low voltage swing at the output load so that the circuit can be designed for high linearity. The transimpedance amplifier, preferably an operational amplifier with a negative feedback resistor having its input nodally connected to the output load, senses the current flow in the output load to provide an output voltage of a desired magnitudes. Problems of parasitic capacitance are thus avoided.

As preferred the third transistor has an emitter load comprising a degeneration resistor in series with the collector emitter path of a fourth transistor. Preferably a balanced configuration is provided as shown in figure 1 with an identical set of transistors operating in antiphase.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of a known mixing circuit; and, Figure 2 is a circuit diagram of the circuit of Figure 1 incorporating the present invention.

Referring now to Figure 2, similar parts to those shown in Figure 1 will be denoted by identical reference numbers.

In Figure 2, a transpedance amplifier 10 is nodally connected to the output load XL, without any intermediate impedance. Amplifier 10 comprises an operational amplifier 12 with a negative feedback resistor 14 connected to the inverting input. The non-inverting input is grounded. Thus the output V0 of the amplifier is proportional to the current io flowing in the feedback resistance 14. The output load XL may be regarded as connected to a virtual earth and hence the signal swing across XL will be very small (for an ideal operational arnplifier the swing will be zero). The gain of this configuration can be shown to be 2RElxR14 and is independent of available supply voltage. Problems of parasitic capacitance are avoided in view of the presence of the virtual earth.

Claims (6)

CLAIMS:

1. A mixing circuit comprising first and second transistors connected in differential configuration having as an emittter load a third transistor, a first signal being applied to the base of the first transistor and a second signal being applied to the base of the third transistor, whereby to provide a signal across an output load of the first transistor proportional to the product of the first and second signals, and a transimpedance amplifier being coupled to said output load to provide a voltage output signal dependent on the current flow in said output load.

2. A mixing circuit according to claim 1 wherein said transimpedance amplifier comprises an operational amplifier with a negative resistive feedback path, having an input nodally connected to the output load.

3. A mixing circuit according to claim 2, wherein said input comprises the inverting input, and the non-inverting input of the operational amplifier is connected to ground.

4. A mixing circuit according to any preceding claim including a resistor in the emitter path of the third transistor.

5. A mixing circuit according to claim 1 including fourth and fifth transistors connected in differential configuration having as an emitter load a sixth transistor, said first signal being applied to the base of the fourth transistor and an antiphase version of the second signal being applied to the base of the sixth transistor, the third and sixth transistors having a common emitter load, and the output of the fifth transistor being coupled to said output load.

6. A mixing circuit substantially as described with reference to figure 2 of the accompanying drawings.