交流放大电路如图所示。电容对信号来说可视为短路。其交流电压增益表达式为________。<br/> <img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/b26c655dbfa342639f2a8d852446aed0.jpeg?x-oss-process=image/sharpen,200"/>