Thin-Film Auto-Balance Diode Gate for Voice-Band Sample and Hold and Resampler Circuits

A thin-film hybrid integrated circuit autobalanced diode transmission gate used in a voice-band pulse-code modulation (PCM) terminal will be described. The gate circuit is designed for sample and hold and resampler operations prior to digitalization and after decoding. As in the sampling circuits previously designed for an experimental high-speed high-performance PCM terminal, the diode quad gate is switched by a current routing emitter-coupled transistor pair. In this configuration the diodes are effectively current switched, resulting in extremely low distortion of the signal samples. With the most commonly used voltage-controlled diode quad gate, subnanosecond switching speeds are required to obtain the same linearity obtained here with much lower switching speeds. In the voice-band coding application, any appreciable gate output voltage offset can impair the overall coder-decoder S/N ratio for low level signals. A novel circuit arrangement has been incorporated into the basic current-routed diode gate circuit, which reduces the output voltage offset normally associated with circuits of this type to very low levels. Although the particular circuit to be described is only moderately fast, a similar configuration having all the features, using higher speed beam leaded transistors and diodes available, would switch in the order of a nanosecond.