20/11/2020 Part 1A - Analysis of Circuits Cambridge University Engineering Department Michaelmas Term 2020 Prof. T. D. Wilkinson (tdw13@cam.ac.uk) Handout 2 - Analysis of Circuits and Devices (We wish you a merry Christmas ... and a gappy New Year) 1 DC Voltages (Capital Letters) ID 1 Input = VGS IG=0 G VG = VGS OK + RG - RD D ÃŽVDS S V DD Output = V DS Gate bias voltage source With no input connected, choice of VGs VDs and Ip select operating point. 2 1
20/11/2020 Example: Using the circuit above, but with a n-channel enhancement MOSFET, we would like an operating point of VGS = 1.5V, and ID = 2.2 mA, given a supply voltage, VDD = 5V. We don't have all the information we need yet (we don't know VDs or RD, we need to use the load-line method. We know VDD = 5V, hence we can set: V DS = D = 2 . 5V V DD From the intersection of the load-line with the VGs = 1.5V characteristic, we find the operating point, P. ID= VDD - V DS R D 3 ID (mA) Load-line VDD RT = 4.3 mA 3 2 VGS = 2V VGS= 1.5V Operating Point (P) 1 VGS = 1V 0V 1 3 DD 2 5 V DD VDS 4 2
20/11/2020 Hence from the y intercept we can obtain the value of Rp and also the value of the voltage source VG which sets the suitable VGS. Don't forget RG. 01-0 00 =1.14152 In = VG=1.5V,RG=1MQ2 Rp =1.2 kQ Standard E12 value Project Point Project Kuing Projaar Putit 5 Now we can estimate the gain of the circuit graphically. The input is set by the DC source VG which sets the operating point. Any signal coming into the amplifier will add or subtract from this voltage causing the operating point to move up and down the load line. The input voltage gives is the change in VGs (AVGs) and the output is the corresponding change in VDS (AVDS). Gain = NInput AOutput _AV DS AV GS 6 3