An Op-Amp Approach for Bandpass VGAs With Constant Bandwidth

Two approaches to implement variable gain amplifiers based on Miller op-amps are discussed. One has true constant bandwidth while the other has essentially reduced bandwidth variations with varying gain. Servo-loops and ac coupling techniques with quasi floating gate transistors are used to provide a bandpass response with very low cutoff frequency in the range of hertz. In practice, one of the schemes is shown to have bandwidth variations close to a factor two while the second one has true constant bandwidth over the gain tuning range. Experimental results of test chip prototypes in 180-nm CMOS technology verify the theoretical claims.