Lab 4: Frequency Response of CG and CD Amplifiers.

Transcription

1 ESE 34 Electronics aboratory B Departent of Electrical and Coputer Enineerin Fall 2 ab 4: Frequency esponse of CG and CD Aplifiers.. OBJECTIVES Understand the role of input and output ipedance in deterinin the ain of FET aplifiers: Measure the frequency response of CG aplifier; Measure the frequency response of CD aplifier. 2. INTODUCTION 2.. Frequency response of CG and CD aplifiers. In previous lab we have studied the ain of the CS aplifier payin special attention to the effect of load resistance and capacitance on aplifier bandwidth. By connectin capacitance to the load, we were essentially addin a low-pass filter at the output on CS aplifier. We have intentionally iniized the contribution of the parasitic low-pass-filter at the input to avoid the discussion of associated Miller effect. We did that by takin easured value of the ate-to-source sinal voltae for an input voltae. Strictly speakin the ate aterial of the MOSFET has finite resistance and even in the experiental arraneent of lab 3 one cannot afford to unconditionally foret about low-pass-filter at the input. In this lab we will study that input low-pass-filter. We will also discuss the topoloies of the FET aplifiers that can help to iniize the bandwidth liitations associated with the low-pass-filters either at the input or at the output. Fiure a shows the eneric CS aplifier with realistic sinal source havin finite output resistance. The equivalent circuit in Fiure b can be used to predict hih 3dB frequency f H. In this equivalent circuit the Miller effect is already taken into account and one can see an equivalent capacitance C M in parallel with input capacitance C GS. In our experients we are not usin any G so in our particular case si = si. Equivalent load resistance = (r D ). If open circuit voltae ain is bein easured then = and = (r D ). Fiure. ow-pass-filter at the input of CS aplifier (coposed of si and C GS C M ) will liit the aplifier bandwidth (BW) and correspondin hih 3B frequency can be estiated as: f H 2 π C C 2 π C C. () si GS M si GS GD

2 ESE 34 Electronics aboratory B Departent of Electrical and Coputer Enineerin Fall 2 Miller effect aplifies the role of C GD and it is often the C M rather than C GS that liits the BW of CS aplifier. Coon Gate (CG) aplifier does not suffer fro Miller effect since no capacitive couplin (well, alost no) is present between input and output. Fiure 2a shows CG circuit topoloy for sinals only, i.e. power supplies are oitted but assued. Fiure 2. Fiure 2b shows an equivalent circuit with r inored and b, i.e. back-ate transconductance, shown. In interated circuit (IC) CG aplifiers, ate and back-ate transconductances act toether since source-to-ate and source-to-bulk sinal voltaes are equal to each other (bulk is connected to power supply voltae, i.e. round for sinal). In our lab experients we should not care about b since we often connect bulk to source for siplicity. Effective low-pass-filter at the input of CG aplifier has 3dB cut-off: f H input. (2) 2 π S CGS b The low-pass-filter at the output will have 3dB cut-off at (often doinant pole in any applications): f H output 2 π C GD C. (3) Here is an equivalent load, i.e. includin D. Both poles due to input and output low-pass-filters are often hiher then f H for CS aplifier (see equation ()). Hence, BW of CG aplifier is extended as copared to CS one. However, low input resistance of CG aplifier can reduce idband voltae ain especially for sinal sources with lare internal resistance (observe voltae divider ade of S and /( + b )). Hence we can expect (in our experients for CG aplifier) wider BW but lower idband ain as copared to CS one. In special case when load capacitance C liits the BW of the CS aplifier the chane of the circuit topoloy to CG one would not iprove BW. Product of and C will essentially deterine the tie constant of the output low-pass-filter for both CS and CG aplifiers. However, one can use voltae buffer with reduced output resistance. Naely, one can use Coon Drain (CD) confiuration. Fiure 3a shows scheatically an IC realization of CD aplifier circuit. Aain, in eneral, bulk is on round for sinal, but we will just connect it to source to eliinate the body effect discussion at all. Input is applied to ate and output is collected at source. Drain is rounded for sinals. Equivalent circuit is shown at 2

3 ESE 34 Electronics aboratory B Departent of Electrical and Coputer Enineerin Fall 2 Fiure 3b. The equivalent circuit includes extra eleents, naely, load capacitance C and sinal source internal resistance. Fiure 3. In our lab we will be interested in perforance of CD (source follower) aplifier as a voltae buffer with sall output resistance. Hence we will easure input voltae directly at ate (this is equivalent to havin si = ) and use no to iniize roles of C GS and C GD. Then the hih 3dB frequency will be ainly deterined by CG equivalent output resistance and load capacitance: f H 2 π out C 2 π C. (4) 2 π r C As lon as / is saller than (equation (3)) source follower will have extended BW, i.e. CG confiuration is less sensitive to load capacitance than CS and CG ones. 3. PEIMINAY AB 3.. Assue si = kω, C GS = pf and C GD = pf. Usin paraeters of CS aplifier fro laboratory experients 2 and 3 find the correspondin BW of open-circuit voltae ain Calculate the hih 3 db frequency for open-circuit voltae ain of CG aplifier with the sae bias, D, S = si as CS fro Calculate the hih 3dB frequencies for CS and CD aplifiers each loaded with 3 pf capacitor. Assue = si =. All necessary paraeters take fro your lab 2 and 3 data. 3

4 ESE 34 Electronics aboratory B Departent of Electrical and Coputer Enineerin Fall 2 4. EXPEIMENT (we are interested in hih 3dB frequency, so start easureents fro idband frequencies, i.e. skip easurin the low frequency part of anitude responses to save tie) 4.. Build the CS circuit shown below usin = = 5 V. Select 5 kω potentioeter and adjust it to obtain 25 µa bias current for. For C S and D use values fro lab 2 or 3. Select S = kω. Use oscilloscope to visualize both input and output sinusoidal wavefors. Perfor easureents of the anitude D si M CS Present the easureent results in the for of table. Plot the experiental results. Copare hih 3 db frequency with the value easured in 4. part of lab Adjust your circuit to obtain CG aplifier. Naely, round the ate and connect function enerator throuh kω S and C S to source of (see circuit below). Perfor easureents of the anitude D CS si M Present the easureent results in the for of table. Plot the experiental results. Explain difference in idband ain and hih 3dB frequency between CG and CS aplifiers. 4

5 ESE 34 Electronics aboratory B Departent of Electrical and Coputer Enineerin Fall Asseble CD aplifier. Naely, reove D and connect drain directly to V DD, apply input sinal to ate of (see circuit below) and easure output at source. Perfor easureents of the anitude M Present the easureent results in the for of table. Plot the experiental results. Coent on values of idband ain and hih 3 db frequency Auent circuit fro 4.3 with load capacitance C = 3 pf. Perfor easureents of the anitude M C Present the easureent results in the for of table. Plot the experiental results. Explain chane in hih 3dB frequency value. 5. EPOT The report should include the lab oals, short description of the work, the experiental and siulated data presented in plots, the data analysis and coparison followed by conclusions. Please follow the steps in the experiental part and clearly present all the results of easureents. Be creative; try to find soethin interestin to coent on. 5