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
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