Questions on variable gain amplifier circuit in razavi book

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promach

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For the following variable gain amplifier circuit ,

1) Should Vout1 = RDID2 - RDID1 instead ?

2) Why "if I1 = 0, then Vout = +gmRD , and if I2 = 0, then Vout = −gmRD . For I1 = I2 , the gain drops to zero." ?

 

Note that I1 and I2 are the currents through M1 resp. M2 - and NOT the current sources in part b) of the figure.
More that that, notice the definition of Vout: It is the difference between both drain voltages.
 

Although the answer to your question is pretty clear and can be derived from the shown figure 4.41, the relevant figures 4.42a and 4.42b (apparently showing the summation of both diff-pair outputs) are missing in your post. That makes me wonder if you actually read the text?
 

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