Driver or buffer circuits are often used to help isolate outputs and match output impedance to expected load impedances. An example of one known differential output driver 100 is illustrated in FIG. 1. The driver 100 includes a pair of transistors Q1, Q2 in a push-pull configuration driving a resistive load RL. A direct current (DC) bias circuit 102 maintains a constant DC current through the transistors Q1, Q2. The DC bias circuit includes a pair of transistors M1, M2 with gates tied to the gate of a transistor M0 driven by a current source I0. Unfortunately, this type of driver can generate undesirable amounts of distortion.
As the input voltages Vp and Vm change during operation of the driver 100, the output voltages of Q1 and Q2, Vop and Vom, respectively, change. In this example, the current through RL may be calculated as IL=(Vop−Vom)/RL. The change in load current IL is accommodated by a change in the current through the transistors Q1 and Q2 because the DC bias circuit 102 maintains a fixed Ip2 and Im2. As shown in FIG. 2, when Vop−Vom is at a maximum, Ip1 is at a maximum and Im1 is at a minimum. Similarly, when Vop−Vom is at a minimum, Im1 is at a maximum and Ip1 is at a minimum. The current swing of Ip1 and Im1 is offset by Ip2 and Im2.
The modulation index for this swing in transistor current may be defined as IQ(max, min)/IQ(DC), where IQ (max, min) is the maximum or minimum total current through the transistor (i.e., bias current +/−load current) and IQ(DC) is the bias current through the transistor, which is substantially constant. Even a small change in load current IL may lead to distortion because the change in load current is reflected by a proportional change in transistor current for the output transistors Q1, Q2. The larger the load current swing, the larger the modulation index, and the larger the distortion. This is because the transconductance of Q1 and Q2 varies as the current through these transistors varies. A variation in transconductance results in a variable gain because the base-emitter voltage drop in Q1 and Q2 varies with changes in transistor current.
One way to reduce the modulation index in the driver of FIG. 1 is to increase the DC bias current. By increasing the DC bias, the significance of variations in the load current can be reduced. Although the modulation index can be reduced in this manner, it merely masks the continuing problem of the distortion in the driver 10. Also, reducing distortion by increasing DC bias involves greater power consumption and may be unacceptable in low power applications.