CASCADED BJT AMPLIFIER Cascode connection A cascode connection

CASCADED BJT AMPLIFIER Cascode connection - A cascode connection has one transistor on top of (in series with) another - The i/p applied to a C-E amp. (Q 1) whose output is used to drive a C-B amp. (Q 2) 1

CASCADED BJT AMPLIFIER Cascode connection - The o/p signal of Q 1 becomes the i/p signal of Q 2 2

CASCADED BJT AMPLIFIER Cascode connection - The advantage: provides a high i/p impedance with low voltage gain to ensure the i/p Miller capacitance is at a min. with the C-B stage providing good high freq. operation 3

CASCADED BJT AMPLIFIER Cascode connection DC analysis May be performed using the following figure; 4

CASCADED BJT AMPLIFIER Cascode connection The equations are (assuming VBE = 0. 7 V for both BJT’s); 5

CASCADED BJT AMPLIFIER Cascode connection And; The above equations may solved for the two unknown currents namely I 1 and IB 1. 6

CASCADED BJT AMPLIFIER Cascode connection AC analysis The equivalent circuit under AC condition 7

CASCADED BJT AMPLIFIER Cascode connection The ac equivalent circuit using hybrid- model for BJT 8

CASCADED BJT AMPLIFIER Cascode connection 9

CASCADED BJT AMPLIFIER Cascode connection At node E 2; Or; 10

CASCADED BJT AMPLIFIER Cascode connection Substituting in (1); 11

CASCADED BJT AMPLIFIER Cascode connection The small-signal voltage gain; When 2 >> 1 12

CASCADED BJT AMPLIFIER Cascode connection Example 4 Compute the approximate smallsignal voltage gain 13

CASCADED BJT AMPLIFIER Example 4 – Solution DC analysis The circuit under DC condition 14

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) 15

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Substituting values; 16

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Or; 17

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Substituting values; Therefore; 18

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Substituting for I 1 in (1) 19

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) 20

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) AC analysis Small-signal equivalent circuit using hybrid- model 21

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Another form of small-signal equivalent circuit using hybrid- model 22

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) At node E 2; 23

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Hence; Substituting for v 2 in (1); Or; 24

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Or; The voltage gain; When 2 1; 25

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Substituting values; 26