- The amplifier operates as in figure (a).
- In sampling mode, switches S1 and S2 are ON and S3 is OFF creating a virtual ground at X and allowing voltage across C1 to track input voltage as in figure (b).
- At the end of sampling mode, switch S2 is OFF which injects a constant charge Δ$q_2$ onto node X.
- Subsequently, switch S1 turns OFF and switch S3 turns ON as in figure (c).
- Since, Vp goes from Vin to 0 and output voltage changes from 0 to Vin0 (C1/C2) providing a voltage gain of (C1/C2).
This circuit is called “non-inverting” because final output has same polarity as Vin0 and the gain can be greater than unity.
As with the unity-gain circuit of Fig (a), the non-inverting amplifier avoids input-dependent charge injection by proper timing, i.e, turning S2 off before S1 (Fig. d). After S2 is off, the total charge at node X remains constant, making the circuit insensitive to charge injection of S1 or charge "absorption" of S3.