BJT Common-Base (CB) Configuration Characteristics and Switching Times
Unit 3: BJT Common-Base Configuration
2. Draw the Circuit Diagram and Explain the Characteristics of BJT in CB
Characteristics of BJT in CB Configuration
The V-I characteristics of a BJT in the common-base (CB) configuration describe the relationship between the voltages and currents at the emitter and collector for various base currents. These characteristics are divided into input and output characteristics.
1. Input Characteristics (Emitter–Base V‑I)
These characteristics show the relationship between the emitter current (IE) and the emitter–base voltage (VEB) for a constant collector–base voltage (VCB).
- Region of operation: The input characteristic curve is similar to a diode V‑I curve because the emitter–base junction is a PN junction.
- Active region: When VEB exceeds the threshold voltage, the emitter–base junction becomes forward biased and IE increases exponentially with VEB.
2. Output Characteristics (Collector–Emitter V‑I)
These characteristics show the relationship between the collector current (IC) and the collector–emitter voltage (VCE) for various levels of emitter current (IE).
Regions of operation:
- Cut-off region: When VEB < 0.7 V (for silicon transistors), the transistor is off. Both IE and IC are nearly zero.
- Active region: The emitter–base junction is forward biased and the base–collector junction is reverse biased. Here, IC is primarily controlled by IE and is nearly independent of VCB. The relationship can be expressed as IC ~ αIE, where α (current gain in CB configuration) is close to 1 but slightly less.
- Saturation region: Both the emitter–base and base–collector junctions are forward biased. In this region, IC increases with VCE but is limited and does not increase significantly.
Summary of Output Regions
- Cut-off region: VEB < 0.7 V (for silicon transistors), IC ≈ 0, IE ≈ 0.
- Active region: VEB ≥ 0.7 V (for silicon transistors), IC ≈ αIE, VCE relatively high.
- Saturation region: VCE low, IC and IE both large; IC is not strictly proportional to IE.
Graphical Representation
1. Input characteristics (IE vs VEB for a given VCB):
- Shows an exponential increase in IE with VEB after the threshold voltage (approximately 0.7 V for silicon transistors).
2. Output characteristics (IC vs VCE for various IE):
- Cut-off region: Near zero IC regardless of VCE.
- Active region: IC ≈ αIE and remains approximately constant for a given IE as VCE increases.
- Saturation region: IC is limited and does not increase significantly with VCE.
Explanation of Regions
- Cut-off region: The transistor is off. VEB is too low to turn it on, so both IE and IC are negligible.
- Active region: The transistor is on and operating normally. The emitter–base junction is forward biased and the base–collector junction is reverse biased. IC is controlled by IE and is approximately equal to αIE.
- Saturation region: The transistor is fully on. Both junctions are forward biased and IC is at its maximum for the given IE.
3. Explain Transistor Switching Times in Detail
Transistor switching times are critical parameters that define how quickly a transistor can transition between its on (saturation) and off (cut-off) states. These parameters are essential for applications involving digital circuits, high-speed switching, and pulse‑width modulation.
Key Switching Time Parameters
Important switching times include delay time, rise time, storage time, and fall time. Each describes a specific interval in the transistor’s response to input changes.
Delay Time (td)
Definition: The time interval between the application of an input signal (base current) and the beginning of a significant change in the output signal (collector current).
Explanation: During this time, the transistor starts to respond to the input signal, but the output current has not yet begun to rise significantly. The delay time is associated with charging the base–emitter junction capacitance and initiating the forward bias of the base–emitter junction.
Rise Time (tr)
Definition: The time interval during which the output current rises from 10% to 90% of its maximum value.
Explanation: This period corresponds to the transistor switching from the off state (cut-off region) toward the on state (saturation region), and reflects how quickly the device can turn on after the delay time.
Storage Time (ts)
Definition: The time interval during which the input signal has been removed, but the output current remains high before starting to decrease significantly.
Explanation: This delay occurs because excess minority carriers stored in the base region during saturation need time to recombine or be removed. Storage time can limit the turn‑off speed of the transistor.
Fall Time (tf)
Definition: The time interval during which the output current falls from 90% to 10% of its maximum value.
Explanation: This period corresponds to the transistor switching from the on state (saturation region) back to the off state (cut-off region). Fall time, together with storage time, determines how quickly a transistor can turn off.