The present invention relates to voltage controlled attenuator circuits, and a means to provide accurate gain settings for such attenuators. In particular, the invention relates to such circuits for use in integrated circuit speakerphone systems.
Voltage controlled attenuators find applications in a wide variety of systems, to adjust the amplitude of signals passed through such systems according to the magnitude of a control signal. For example, in a speakerphone circuit, attenuators are included in both the transmit and receive channels to provide half duplex communication. The transmit and receive attenuators are typically operated in a complementary manner, while one is at maximum gain the other is at maximum attenuation and vice versa. The setting of each attenuator is adjusted so that the difference between the levels remains the same. Using this technique a constant loss is inserted between the two channels and prevents instability that would otherwise occur, due to signal coupling between the loudspeaker and microphone, or sidetone through a hybrid circuit. One requirement of attenuators used in a such an arrangement is accurate gain settings to ensure consistent performance of the speakerphone system. Large variations of the gain level of each attenuator makes the inserted loss between the two channels unpredictable and presents difficulties maintaining the stability of the system.
In a typical speakerphone, the gain setting of a channel is dependent upon the detection of speech within that channel. If a far end talker is speaking, the receive signal is greater than the transmit signal, and the transmit attenuator should be set to maximum loss while the receive attenuator is set to maximum gain. The reverse is true if the mirror-end talker is speaking. By monitoring the amplitudes of the signals in both channels, a control circuit may be developed to determine which channel is active and adjust the gains accordingly. A further requirement of an attenuator used in a speakerphone is the need to minimize the feedthrough of control signal into the audio path. Feedthrough generates an audible "thump" in the speech channel which occurs when switching from one channel to the other. With sufficient magnitude, the feedthrough causes errors in switching due to its detection as a false speech signal. The primary source of feedthrough is gain dependent offset within the attenuator.
FIG. 2 illustrates an example of a prior art voltage controlled attenuator. An input voltage is provided at a point 210 through a resistor R2 to a first amplifier 212. Amplifier 212 operates to sink the current from emitter coupled transistors 218,220 and 214, 216 which receive current from a current source 222. The output is provided through a second amplifier 224 to a voltage output 226. A control voltage indicated VC is applied to transistor 214 and transistor 220. The control voltage VC affects the two transistors in opposite ways, one being NPN, the other PNP. In response to an increasing control voltage, the current in transistor 214 will increase while the current in transistor 220 will decrease. Thus, the current through the two legs of the amplifier section will split in proportion with the control voltage. In accordance with this varying current, the voltage amplification between the input at point 210 and the output at point 226 will vary, such that the gain of the attenuator is directly proportional to the split of the current from current source 222.
One problem with a circuit such as that shown in FIG. 2, when used in a typical bipolar integrated circuit, is the different characteristics of the NPN and PNP transistors, which causes a DC feedthrough from the input to the output and which varies in accordance with the level of amplification set by the control voltage. It would be desirable to have a voltage controlled attenuator which eliminates this DC feedthrough effect due to the use of both NPN and PNP transistors in the current splitting core.