Lecture 28: MOSFET as an Amplifier. Small-Signal Equivalent Circuit Models.

Transcription

1 hites, EE 320 ecture 28 Page 1 of 7 ecture 28: MOSFET as a Amplifier. Small-Sigal Equivalet Circuit Models. As with the BJT, we ca use MOSFETs as AC small-sigal amplifiers. A example is the so-called coceptual MOSFET amplifier show i Fig. 7.2: i v S (Fig. 7.2) This is oly a coceptual amplifier for two primary reasos: 1. The bias with V GS is impractical. (ill cosider others later.) 2. I ICs, large valued resistors take up too much physical space. (ould use aother triode-regio biased MOSFET i lieu of R.) To operate as a small-sigal amplifier, we bias the MOSFET i the saturatio regio. For the aalysis of the C operatig poit, we set vgs 0 so that from (7.25) with 0 1 i 2 k vgs Vt (7.25),(1) 2017 Keith. hites

2 hites, EE 320 ecture 28 Page 2 of 7 From the circuit VS V IR (7.26),(2) For operatio i the saturatio regio vg Vt vgs vs Vt or vs vgs Vt (3) where the total drai-to-source voltage is vs V S v ds bias AC Similar to what we saw with BJT amplifiers, we eed make sure that (3) is satisfied for the etire sigal swig of v ds. ith a AC sigal applied at the gate vgs VGS vgs (7.27),(4) Substitutig (4) ito (1) i k VGS vgs Vt k VGS Vt vgs (5) k GS t VGS Vt vgs k vgs (7.28),(6) (C) (time varyig) k V V 2 + I The last term i (6) is oliear i v gs, which is udesirable for a liear amplifier. Cosequetly, for liear operatio we will require that the last term i (6) be small with respect to the liear term, which is the secod term ad is proportioal to v gs : 1 2 k vgs k VGS Vt vgs

3 hites, EE 320 ecture 28 Page 3 of 7 or v 2 V V (7.29),(7) gs GS t If this small-sigal coditio (7) is satisfied, the from (6) the total drai curret is approximately the liear summatio i I i d (7.31),(8) C AC where id k VGS Vt vgs. (9) From this expressio (9) we see that the AC drai curret i d is related to v gs by the so-called trasistor trascoductace, g m : id gm k VGS Vt [S] (7.32),(10) v gs which is sometimes expressed i terms of the overdrive voltage VOV VGS Vt gm k VOV [S] (7.33),(11) Because of the V GS term i (10) ad (11), this g m depeds o the bias, which is just like a BJT. Physically, this trascoductace g m equals the slope of the i - v GS characteristic curve at the Q poit: i gm (7.34),(12) v GS v GS V GS

4 hites, EE 320 ecture 28 Page 4 of 7 (Fig. 7.11) astly, it ca be easily show that for this coceptual amplifier i Fig. 7.2, vds Av gmr (7.36),(13) v gs Cosequetly, Av gm, which is the same result we foud for a similar BJT coceptual amplifier [see (7.78)]. MOSFET Small-Sigal Equivalet Models For circuit aalysis, it is coveiet to use equivalet smallsigal models for MOSFETs, as it was with BJTs. I the saturatio mode, the MOSFET acts as a voltage cotrolled curret source. The cotrol voltage is v gs ad the output curret is i d, which gives rise to this small-sigal model:

5 hites, EE 320 ecture 28 Page 5 of 7 i 0 g i d V I A 1 I i s (Fig. 7.13b) Thigs to ote from this small-sigal model iclude: 1. ig 0 ad vgs 0 ifiite iput impedace. 2. r o models the fiite output resistace. Practically speakig, it will rage from 10 k 1 M. Note that it depeds o the bias curret I. 3. From (10) we foud gm k VGS Vt (14) Alteratively, it ca be show that 2I I gm (7.42),(15) VOV ( VGS Vt )/2 which is similar to gm IC VT for BJTs. Oe big differece from BJTs is VT 25 mv while VGS Vt /2 0.1V or greater. Hece, for the same bias curret g m is much larger for BJTs tha for MOSFETs. A small-sigal T model for the MOSFET is show i Fig. 7.17:

6 hites, EE 320 ecture 28 Page 6 of 7 i d i 0 g i s (Fig. 7.17a) Notice the direct coectio betwee the gate ad both the depedet curret source ad 1/g m. hile this model is correct, we ve added the explicit boudary coditio that i g = 0 to this small-sigal model. It is t ecessary to do this because the currets i the two vertical braches are both equal to gv m gs, which meas ig 0. But addig this coditio ig 0 to the small-sigal model i Fig. 7.17a makes this explicit i the circuit calculatios. (The T model usually shows this direct coectio while the model usually does t.) MOSFETs have may advatages over BJTs icludig: 1. High iput resistace 2. Small physical size 3. ow power dissipatio 4. Relative ease of fabricatio.

7 hites, EE 320 ecture 28 Page 7 of 7 Oe ca combie advatages of both techologies (BJT ad MOSFET) ito what are called BiCMOS amplifiers: (Sedra ad Smith, 5 th ed.) Such a combiatio provides a very large iput resistace from the MOSFET ad a large output impedace from the BJT.