sconductance of this first stage. The mixer block diagram was then extracted from the nodal equations of the topology in Fig. 4.25 and is shown in Fig. 4.26. The time-varying subblocks are indicated with a tilde on top of the block name. The two main sources of time-varying behavior are due to the modulation of the small-signal parameters of the input transistors and of the transconductance of the second OTA stage. As the input transistors are concerned, use of equation (4.86) yields the small-signal current model (4.87) Here, the capital letters represent the (time-varying) operating points while the small letters represent the small-signal components. The first term in (4.87) captures the de- sired mixing behavior, corresponding to the block Hosc(s in Fig. 4.26. The last term in (4.87) adds a time-varying component to the OTA input admittance. This corresponds to the feedback path Hn (s). More precisely, Hosc(s) corresponds to the series con- nection of a multiplication with (Vosc(t) - Vn1(t)) and an LTI filtering operation with transfer function Yib(s) Furthermore, Hnl (s) is the series connection of - -BVn1 (t) and 4.3.7 Application 1: linear downconversion mixer Y16(s)/ Yo(s) Hose(s) Hi(s) A1(s) A2(s) H2(s) Figure 4.26: Block diagram of the linear downconversion mixer. Time- varying blocks are marked with a tilde on top of their name. Blocks without a tilde are time-invariant. Yfb(s) In the above, Vosc(t) = Vosc cos (wot) is modeled as a pure cosine. Note that, in the expressions that follow, the symbol Vosc is always used to refer to the oscilla- tion amplitude. When referring to the complete oscillation waveform, we'll explicitly denote the time dependency using Vosc(t). The second source of time-varying behavior is, as was already explained before, due to the modulation of the small-signal transconductance of the second OTA stage, or (4.88) This modulated small-signal transconductance is symbolically represented by the Fourier coefficients gm2,k In the block diagram of Fig. 4.26, it is contained within th