cascode | PartQuest™ Explore

Under development Compare Cascode vs. Single BJT Transistor Amplifier

Mike Donnelly
Designer19

Mike Donnelly

Member for

10 years 5 months

1,555

designs

10

groups

Member of the PartQuest Explore Development Team. Focused on modeling and simulation of analog, mixed-signal and multi-discipline systems covering a broad range of applications, including power electronics, controls and mechatronic systems.

Description

This example shows the bandwidth improvement that results from a cascode amplifier configuration (right-side circuit), compared to a single transistor amplifier configuration (left side). In these circuits, identical transistor models are used. They are ideal models that have no internal Miller capacitance or Ccb, The Miller capacitance is represented by an external 10 pF discrete capacitor for each device.In the single transistor circuit, the effective value of the Miller capacitance is increase by the current gain of the transistor, hfe = 100. So the RC time constant of the load resistance with that increased capacitance value, reduces the amplifier bandwidth by almost the same factor.For the cascode amplifier, the voltage at the collector of the CE transistor q2 does not change significantly, so the Miller effect is largely suppressed for that device. The CB transistor q3 acts as a simple current follower. Because the current in the Miller capacitance of q3 is drawn from the low impedance bias circuit, it does not significantly impact the base current of q3.You can move the probes around and look at the time-domain or frequency-domain signals at any point in the circuit, to gain more understanding of these component interactions.

About text formats

Terms of Use Cookie Notice Data Privacy Notice

Under development Compare Cascode vs. Single BJT Transistor Amplifier

Mike DonnellyDesigner19

Mike Donnelly

Member for

Mike Donnelly
Designer19