The objective of this lab activity is to investigate the folded cascode configuration of a multistage (two transistor) amplifier.
As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current -V is added as in CA-V or when configured to force current / measure voltage -I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage -H is added as CA-H.
Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.
A conventional combination of a common emitter amplifier with a stacked common base ( cascode ) stage using the same type device (i.e. both NPN or both PNP ) requires at least two VBE and two VSAT of supply headroom. This can limit the available signal swing for low supply voltages. The folded cascode which uses one NPN and one PNP device requires as little as one VBE and one VSAT of supply headroom. This allows wider signal swing operation than the stacked cascode / common emitter configuration. In figure 1 PNP Q1 forms an emitter degenerated common emitter amplifier with emitter resistor R1, and collector resistor R2. NPN Q2 acts as a common base or cascode stage with its emitter connected to the collector of Q1.
ADALM1000 hardware module
1 - small signal PNP transistor, Q1 ( 2N3906 or similar )
1 - small signal NPN transistor, Q2 ( 2N3904 or similar )
3 - 1 KΩ resistors
Build the two transistor folded cascode amplifier circuit shown in figure 1 on your solder-less breadboard.
Figure 1, Folded Cascode Circuit
The channel A voltage generator should be configured for a 100 Hz Sine wave with 3.0 volt Max value and 2.0 V Min value. The channel B scope input, CB-H, is used to alternately measure the voltage at the collector and emitter of Q2 and the emitter of Q1. To compare these waveforms at the same time use the Snap-Shot option to save and display reference waveforms. To measure the input to output gain, the CB-V / CA-V Math waveform along with the input offset feature can be used.
Using the measurement plots and data taken calculate the input to output voltage gain.
The voltage at the emitter of Q1 will be 180 degrees out of phase with the voltage at the collector of Q2. Using the DC shape for CA-V adjust its value until the two voltages are equal, i.e. the voltage across R1 is equal to the voltage across RL. How does the voltage across R1, RL relate to the voltage across R2?
Place another 1 KΩ resistor in parallel with R2. Repeat the above tests and explain any differences you measure.
What is the maximum peak to peak output voltage swing that can be produced with the folded cascode stage as shown in figure 1? How does that relate to the constant voltage applied to the base of Q2?
What limits the possible maximum and minimum output voltage seen at the collector of Q2 and why?
What limits the possible maximum and minimum input voltage at the base of Q1 and why?
For Further Reading:
Return to ALM Lab Activity Table of Contents