EMT 112 / 4 ANALOGUE ELECTRONICS Lecture II Multistage Amplifiers [Cascode Connections] March 18, 2008 (Week XIII) 1200 – 1400 DKQ 1

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. (Q1) whose output is used to drive a C-B amp. (Q2)

CASCADED BJT AMPLIFIER Cascode connection The o/p signal of Q1 becomes the i/p signal of Q2

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

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

CASCADED BJT AMPLIFIER Cascode connection The equations are (assuming VBE = 0.7 V for both BJT’s);

CASCADED BJT AMPLIFIER Cascode connection And; The above equations may solved for the two unknown currents namely I1 and IB1.

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

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

CASCADED BJT AMPLIFIER Cascode connection

CASCADED BJT AMPLIFIER Cascode connection At node E2; Or;

CASCADED BJT AMPLIFIER Cascode connection Substituting in (1);

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

CASCADED BJT AMPLIFIER Cascode connection Compute the approximate small-signal voltage gain Example 4

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

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

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

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

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

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) Substituting for I1 in (1)

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

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

CASCADED BJT AMPLIFIER Example 4 – Solution (cont’d) At node E2;

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

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

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