The purpose of this activity is to investigate the common emitter configuration using the BJT device.
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.
ADALM1000 hardware module
4 - Resistors
1 - 50 KΩ Variable resistor, potentiometer
1 - small signal NPN transistor (2N3904)
The configuration, shown in figure 1, demonstrates the NPN transistor used as a common emitter amplifier. Output load resistor RL is chosen such that for the desired nominal collector current IC, approximately one half of the +5 V voltage (2.5 V) appears at VCE. Adjustable resistor Rpot along with Rb sets the nominal bias operating point for the transistor (IB) to set the required IC. Voltage divider R1/R2 is chosen to provide a sufficiently large attenuation of the input stimulus from waveform generator channel A. This is done to more easily view the channel A generator signal, given the rather small signal that will appear at the base of the transistor, VBE. The attenuated CA-V generator signal is AC coupled into the base of the transistor by the 4.7 uF capacitor so as not to disturb the DC bias condition.
Figure 1 Common emitter amplifier configuration
The channel A waveform generator output CA-V should be configured for a 1 KHz sine wave with 3 volt Max and 0 volt Min (3 V p-p). Scope channel CB-H is used to measure alternately the waveform at the base and collector of Q1.
The voltage gain, A, of the common emitter amplifier can be expressed as the ratio of load resistor RL to the small signal emitter resistance re. The transconductance, gm, of the transistor is a function of the collector current IC and the so called thermal voltage, kT/q which can be approximated by around 25 mV or 26 mV at room temperature.
The small signal emitter resistance is 1/gm and can be viewed as being in series with the emitter. Now with a signal applied to the base the same current (neglecting base current) flows in re and the collector load RL. Thus the gain A is given by the ratio of RL to re.
The purpose of this section is to investigate effect of adding negative feedback to stabilize the DC operating point.
Figure 2 Self Biased configuration
How does adding negative feedback help to stabilize the DC operating point
The purpose of this section is to investigate effect of the addition of emitter degeneration.
1 - 5KΩ Variable resistor, potentiometer (500Ω if one is available)
Disconnect the emitter of Q1 from ground and insert RE, a 5KΩ potentiometer, as shown in the following diagram. Adjust RE while noting the output signal seen at the collector of the transistor.
Figure 3 Emitter degeneration added
What effect does adding RE have to the DC operating point of the circuit and how much would you need to adjust Rpot to return the circuit to the same DC bias (IC) you had in figure 1?
What is the effect on the voltage gain, A, by increasing RE?
Adding the emitter degeneration resistor has improved the stability of the DC operating point at the cost decreased amplifier gain. A higher gain for AC signals can be restored to some extent by adding capacitor C2across the degeneration resistor RE as shown in figure 4.
Figure 4 C2 added to increase AC gain
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