| 摘要 |
A variable gain amplifier is described having two parallel amplifying paths driving a common load RL. One path defined by the transistor (Q3) and including a relatively low impedance (L1) provides a high gain/low linearly amplification. The other path defined by the transistor (Q6) by using a relatively high impedance (L2) provides a low gain high linearly amplification. Current generators (I1 and I3) control the DC bias supplied to the bases of respective transistors (Q2 and Q6). By adjusting the two current generators (I1 and I3) in opposite senses, the ratio of the impact that each transistor (Q3 and Q6) has on the load RL can be varied. The provision of mirror image amplifiers using transistors (Q3 and Q7) which share the impedances (L1 and L2) with respective transistors (Q2 and Q6) enables the input and output impedances of the amplifier to remain substantially constant irrespective of the gain being achieved. <IMAGE> |
