12.9
BJT Amplifiers
Bipolar Junction Transistors (BJTs) are pivotal components in amplifier circuits, functioning as voltage-controlled current sources in their active re…
In the active region of a common emitter NPN BJT, the emitter current is the sum of the base current and the collector current. Here, the base voltage controls the collector current in the BJT.
The output voltage across the collector is derived by subtracting the voltage drop across the load resistance from the supply voltage, producing an inverted output relative to the supply voltage.
The relation between collector voltage and base voltage gives the voltage-transfer characteristic curve of the common-emitter BJT amplifier.
It highlights the amplifier's active region, characterized by a steep slope for maximum gain but a nonlinear output voltage.
By applying a base-emitter DC voltage bias at the quiescent point within the active region, the BJT typically achieves nearly linear amplification.
When a small, time-varying signal is applied to the amplifier, it is superimposed on the DC bias voltage around the quiescent point.
So, the BJT operates within a short, nearly linear segment of the characteristic curve, resulting in an amplified output collector voltage.
View the full transcript and gain access to JoVE Core videos
Q1: How does a BJT control collector current in the active region?
In the active region, the base voltage controls the collector current of a bipolar junction transistor. The emitter current equals the sum of base current and collector current. This voltage-controlled current behavior allows the BJT to function as an efficient amplifier, converting weak input signals into stronger output signals through transconductance conversion into voltage amplification.
Q2: Why is the output voltage inverted in a common-emitter BJT amplifier?
The output voltage is inverted because it is derived by subtracting the voltage drop across the load resistance from the supply voltage. This inversion is a hallmark characteristic of common-emitter amplifiers. The inverted output voltage is essentially the input voltage shifted by the supply voltage, producing the opposite polarity relative to the input signal.
Q3: What does the voltage-transfer characteristic curve reveal about BJT amplifier performance?
The voltage-transfer characteristic curve shows how output collector voltage varies with input base voltage changes. The curve's steep slope in the active region indicates maximum gain, but this region also introduces nonlinearity to the output voltage. Understanding this curve helps engineers identify optimal operating points for amplification.
Q4: How does DC bias at the quiescent point enable linear amplification?
A base-emitter DC voltage bias is set at the quiescent point within the active region to establish stable, linear amplification. When a small, time-varying signal is applied on top of this DC bias, the BJT operates within a short, nearly linear segment of the characteristic curve, resulting in amplified output collector voltage with minimal distortion.
Q5: What is the relationship between load resistance and output voltage in BJT amplifiers?
The output voltage is determined by channeling the output current through a load resistor. The voltage drop across this load resistor is subtracted from the supply voltage to produce the final output. This relationship demonstrates how transconductance is converted into voltage amplification, making the load resistance critical for controlling gain.
Q6: How does a small time-varying signal achieve amplification around the quiescent point?
A small, time-varying signal is superimposed on the DC bias voltage at the quiescent point. The BJT then operates within a short, nearly linear segment of the characteristic curve, where the steep slope provides maximum gain. This configuration ensures the amplified output collector voltage maintains signal integrity while achieving significant amplification.
Q7: Why is the active region preferred for BJT amplifier operation?
The active region is preferred because it provides a steep slope on the voltage-transfer characteristic curve, enabling maximum gain. Although this region introduces some nonlinearity, proper DC biasing at the quiescent point allows the BJT to operate within a nearly linear segment, achieving stable amplification with minimal signal distortion.