1 FET FREQUENCY RESPONSE LOW FREQUENCY

2 LOW FREQUENCY – COMMON SOURCE

3 Input RC circuit Output RC circuit Bypass RC circuit Low-frequency equivalent circuit

4 The cutoff frequencies defined by the input, output and bypass circuits can be obtained by the following formulas. where R C1 =R Si +R G where R C2 =R D +R L where R C3 =R S ||1/g m Input RC circuit Output RC circuit Bypass RC circuit

5 EXAMPLE Determine the lower cutoff frequency for the FET amplifier. Given K = 0.4mA/V 2, V TN = 1V, = 0

6 Input RC circuit Output RC circuit Bypass RC circuit

7 Since f c in bypass RC circuit, is the largest of the three cutoff frequencies and is separated from the next highest frequency by more than two octaves, the dominant pole approximation is applicable. Hence the low cutoff frequency f H for the amplifier is  f H = f c = Hz

8 LOW FREQUENCY – COMMON GATE Analyse for mid-band gain and lower cut-off frequency

9

10 DC ANALYSIS

11 AC ANALYSIS – MID-BAND

12

13

14

15 AC ANALYSIS – LOW FREQUENCY

16 Effective resistance across C C1 is;

17 Effective resistance across C C2 is; Since the two frequencies are separated by more than two octaves, the dominant pole approximation can be applied. The lower cut-off frequency is the higher of the two frequencies i.e Hz

18 LOW FREQUENCY – COMMON DRAIN Analyse for A M & f L

19 DC ANALYSIS

20 AC ANALYSIS – MID-BAND

21

22 Substituting values;

23 AC ANALYSIS – LOW FREQUENCY

24 The effective resistance across C 1 is;

25 To find the effective resistance across C 2, we apply a voltage source v x as follows;

26 At node S; or

27 The output circuit is simplified to; The effective resistance across C 2 is;

28 By dominant pole approximation, the lower cut-off frequency is;

29 The cut-off frequency is also known as 3-dB frequency or half-power frequency or corner frequency. The lower cut-off frequency is determined by the components ( R and C ) external to the transistor.