common collector

Related Concepts: Darlington Pair

Based on Laboratory Exercise 15: Common Collector Circuit (also known as Emitter Follower) from your manual, here is the revision note.

1) Experiment Snapshot

Experiment Title: Common Collector Circuit (Emitter Follower)

Objective(s):

Calculate and measure DC operating voltages ( $V_{B}, V_{E}, V_{C}$ ).
Measure voltage gain ( $A_{V}$ ) with and without load.
Determine the phase relationship between input and output signals.

Core Principle / Theory:

Configuration: Collector is common to input and output (effectively grounded for AC).
Voltage Gain: Always less than (but close to) unity ( $A_{V} \approx 1$ ).
Impedance: Very High Input Impedance and very Low Output Impedance.
Phase: Input and output signals are in phase (0° shift).
Application: Used primarily as an impedance matching device (buffer).

Key Formula(s):

Voltage Gain: $A_{V} = \frac{e _{o}}{e _{i}}$
DC Voltage Divider: $V_{B} = V_{CC} \times \frac{R 2}{R 1 + R 2}$
Emitter Current: $I_{E} = \frac{V _{E}}{R _{E}}$

2) Apparatus Setup

Connection / Block Diagram:

Input: AF Generator $\to$ Capacitor C1 $\to$ Base of Q1.
Output: Emitter of Q1 $\to$ Capacitor C2 $\to$ Load / Scope.
Biasing: Voltage divider ( $R 1 = 220 k Ω, R 2 = 220 k Ω$ ) sets Base voltage.
Collector: Connected directly to $V_{CC}$ (24V).
Emitter: Connected to ground via $R 3$ ( $10 k Ω$ ).

Precautions:

Load Resistor: Ensure the 200 $Ω$ potentiometer ( $R 4$ ) is initially set for maximum resistance or disconnected as per instructions.
Scope Connections: Connect the ground clip to the bottom rail (circuit ground), not the emitter (which is at non-zero DC potential).

3) Procedure

Part A: DC Operation

Connect circuit (Fig 15-3). $V_{CC} = 24 V$ . Input signal $e_{i} = 0$ .
Measure DC voltages with VOM with respect to ground:
- Base ( $V_{B}$ )
- Emitter ( $V_{E}$ )
- Collector ( $V_{C}$ )

Part B: AC Gain Measurement 3. Set AF Generator to 1kHz Sine Wave. 4. Adjust input to 100mV pk-pk at the Base ( $e_{i}$ ). Use 0.15V AC range on VOM or Scope. 5. Observe Output waveform at the Emitter ( $e_{o}$ ). Ensure no distortion. 6. Measure output peak-to-peak voltage. 7. Calculate Gain ( $A_{V}$ ).

Part C: Output Impedance 8. Connect $R 4$ ( $200Ω$ Potentiometer) across the output (after C2). 9. Adjust $R 4$ until output voltage ( $e_{o}$ ) drops to one-half its original no-load value. 10. Disconnect and measure the resistance of $R 4$ . This value is the Output Impedance ( $Z_{o u t}$ ).

Part D: Phase Shift 11. Use External Trigger (EXT TRIG) on oscilloscope connected to Output. 12. Compare input and output traces. Check if positive cycles start at the same time.

4) Data Taken / Measurements

Table Template:

Parameter	Condition	Instrument	Measured Value	Unit
DC Analysis
$V_{B}$ (Base)	$V_{CC} = 24 V$	VOM	________	Vdc
$V_{E}$ (Emitter)	$V_{CC} = 24 V$	VOM	________	Vdc
$V_{C}$ (Collector)	$V_{CC} = 24 V$	VOM	________	Vdc
AC Analysis
$e_{i}$ (Input)	1kHz Sine	Scope	0.1 (approx)	$V_{pp}$
$e_{o}$ (Output)	Unloaded	Scope	________	$V_{pp}$
$R 4$ (Impedance)	Half-voltage point	Ohmmeter	________	$Ω$

5) Calculations

Theoretical Base Voltage: $V_{B} = 24 V \times \frac{220 k}{220 k + 220 k} = 12 V$
Voltage Gain: $A_{V} = \frac{e _{o}}{e _{i}} \approx \frac{0.1 V}{0.1 V} \approx 1$

6) Results & Outcome

DC Voltages:
- $V_{B} \approx 12 V$
- $V_{E} \approx 11.4 V$ (0.6V drop BE)
- $V_{C} = 24 V$ (Source Voltage)
AC Gain: Measured gain is slightly less than unity (approx 0.98 to 1.0).
Output Impedance: Found to be low, approx 25 to 75 $Ω$ .
Phase: Input and output are In Phase (0° shift).

7) Why This Outcome Happens (Reasoning)

Unity Gain: The Emitter voltage “follows” the Base voltage ( $V_{E} = V_{B} - 0.7$ ). Since the 0.7V drop is constant DC, any AC change in Base is replicated at the Emitter.
Low Output Impedance: The output is taken from the Emitter. Looking into the Emitter, the impedance is roughly $\frac{R _{so u rce}}{β}$ , making it very low compared to $R_{E}$ .
High Input Impedance: The emitter resistor $R_{E}$ is “reflected” to the base multiplied by $β$ ( $\approx β R_{E}$ ), making the circuit essentially invisible to the source (light load).

8) Viva / Lab Test Quick Prep

Q1: Why is this circuit called an “Emitter Follower”? A: Because the output voltage at the Emitter follows the input voltage at the Base almost exactly (magnitude and phase).

Q2: What is the main application of a Common Collector amplifier? A: Impedance matching (buffer) – connecting a high impedance source to a low impedance load.

Q3: What is the theoretical voltage gain? A: Slightly less than 1 (Unity).

Q4: What is the phase relationship? A: In-phase (0 degrees).

Q5: Why is $V_{C}$ equal to $V_{CC}$ ? A: The collector is connected directly to the supply rail (no collector resistor).

Q6: Compare Output Impedance of CE and CC circuits. A: CE has medium/high output impedance (~ $R_{C}$ ), while CC has very low output impedance (Ohms range).

Common Mistakes:

Expecting Amplification: Students often think “Amplifier” means voltage gets bigger. In CC, voltage stays the same; current/power is amplified.
Loading: Connecting the low-resistance pot ( $R 4$ ) too early can drag down the signal if not adjusted correctly.

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