What Is a Diode-Connected PNP Transistor?
A diode connected PNP is a standard PNP bipolar junction transistor (BJT) wired so that its base and collector terminals are shorted together. By tying these two pins, the transistor loses its ability to amplify current in the traditional forward-active sense and instead behaves as a two-terminal PN junction diode. Current flows into the emitter and exits through the common base-collector node.
While you might wonder why anyone would use a transistor when a cheap 1N4148 signal diode is available, the diode-connected configuration is a cornerstone of analog integrated circuit design and discrete precision circuitry. Because the "diode" is actually the base-emitter junction of a BJT, it shares the exact same silicon characteristics, temperature coefficient, and manufacturing batch variations as other transistors in the circuit. This matching is critical for building current mirrors, logarithmic amplifiers, and temperature-compensated bias networks.
Operating Regions and Electrical Characteristics
When the base and collector are shorted, the collector-base voltage (Vcb) is forced to exactly 0V. This keeps the transistor perpetually at the boundary between the forward-active and saturation regions, often referred to as the "diode region." Because Vcb = 0, the base-collector junction is never forward-biased, meaning the transistor cannot enter deep saturation. Furthermore, eliminating the Vcb voltage minimizes the Early effect (base-width modulation), resulting in a highly stable voltage drop.
| Parameter | Symbol | Typical Value | Condition / Notes |
|---|---|---|---|
| Forward Voltage Drop | Vbe (or Vf) | 0.65V - 0.70V | At Ie = 1mA to 10mA |
| Collector-Base Voltage | Vcb | 0.0V | Forced by external short |
| Temperature Coefficient | TC(Vbe) | -2.0 mV/°C | Crucial for thermal compensation |
| Dynamic Resistance | re | ~26Ω at 1mA | Calculated as 26mV / Ie |
| Reverse Breakdown (Emitter-Base) | Vebo | 5.0V - 7.0V | Exceeding this destroys the junction |
How to Select, Bias, and Test the Right Part
Choosing the right transistor for a diode-connected configuration depends on your current requirements and the need for thermal matching. If you are building a discrete current mirror, Q1 (the diode-connected reference) and Q2 (the output transistor) must be the exact same part number, and ideally from the same manufacturing batch, to ensure their Vbe curves match within millivolts.
Safe Default Part Numbers and Ratings
- 2N3906: The universal small-signal PNP. Vceo = 40V, Ic(max) = 200mA, Pd = 625mW. Ideal for milliamp-level current mirrors and logic-level biasing. (ON Semi 2N3906 Datasheet)
- 2N2907A: The higher-current sibling. Vceo = 60V, Ic(max) = 600mA, Pd = 400mW (TO-92) or 1.5W (TO-18). Use when sourcing hundreds of milliamps.
- BC557: The European equivalent to the 2N3906, featuring slightly higher hFE and lower noise, making it excellent for audio preamplifier bias networks. Vceo = 45V, Ic(max) = 100mA.
Biasing Rules
A diode-connected PNP must never be connected directly across a voltage source. The base-emitter junction has a very steep I-V curve; a small increase in voltage beyond 0.7V will cause massive current flow, leading to thermal runaway and a melted package. You must always bias it using a series current-limiting resistor or a constant-current source.
Testing with a Multimeter
To verify a raw PNP transistor before wiring it as a diode, or to troubleshoot an existing diode-connected circuit, use your digital multimeter's (DMM) Diode Test mode.
- De-energize the circuit. If testing in-circuit, remove the transistor or ensure no parallel low-resistance paths exist.
- Forward Bias Test: Place the red (positive) probe on the Emitter and the black (negative) probe on the Base. You should read between 0.600V and 0.750V.
- Collector-Base Test: Move the black probe to the Collector. You should read a similar 0.600V - 0.750V drop (since both are PN junctions to the emitter).
- Reverse Bias Test: Swap the probes (black on Emitter, red on Base/Collector). The meter should read 'OL' (Open Loop).
- Short Check: Measure resistance between Base and Collector. In a raw transistor, it should read high resistance. In your assembled diode-connected circuit, it must read near 0Ω.
Practical Application: PNP Current Mirror Source
The most common application for a diode connected PNP is the reference leg of a BJT current mirror. In this circuit, the diode-connected transistor (Q1) programs the base-emitter voltage required to pass a specific reference current. A second, identical transistor (Q2) shares that exact same Vbe, causing it to "mirror" the current to the load.
Circuit Specifications
- Target Output Current (I_out): ~1.0 mA
- Supply Voltage (Vcc): +12.0V DC
- Transistors: Q1 and Q2 = 2N3906 (matched pair)
Component Values and Wiring Steps
- Calculate the Reference Resistor (R_ref): The voltage across R_ref is Vcc minus the Vbe drop of Q1. Assuming Vbe = 0.65V, the voltage is 12.0V - 0.65V = 11.35V. To get 1.0 mA, R = V / I = 11.35V / 0.001A = 11,350Ω. Select the nearest standard 1% resistor value: 11.3 kΩ (or 11 kΩ for ~1.03 mA).
- Wire the Emitters: Connect the emitters of both Q1 and Q2 directly to the +12V Vcc rail.
- Configure Q1 (The Diode): Short the base and collector of Q1 together. Connect one end of the 11.3 kΩ resistor to this shorted node, and the other end to Ground (GND).
- Wire Q2 (The Mirror): Connect the base of Q2 directly to the base of Q1. Connect the collector of Q2 to your load, and the other side of the load to GND.
- Verify: Power the circuit and measure the voltage drop across the 11.3 kΩ resistor. It should read ~11.35V, confirming ~1 mA of reference current. Measure the current through the load on Q2; it should match within 2-5% (assuming ambient temperature is equal for both transistors).
Frequently Asked Questions
Why use a diode connected PNP instead of a standard 1N4148 signal diode for biasing?
A standard silicon diode like the 1N4148 has a different temperature coefficient and a different absolute forward voltage curve compared to the base-emitter junction of a BJT. If you use a 1N4148 to generate the bias voltage for a PNP current mirror, the temperature drift of the diode will not track the temperature drift of the mirror transistor. As the circuit warms up, the mirror current will drift significantly. By using a diode-connected PNP of the same type, the thermal coefficients cancel out perfectly, maintaining a stable current over a wide temperature range.
How does the Early effect impact a diode connected PNP compared to a normal BJT?
The Early effect (base-width modulation) causes the collector current of a normal BJT to vary slightly with changes in the collector-emitter voltage (Vce). In a diode-connected configuration, the collector is tied to the base, forcing Vcb to exactly 0V. Because the voltage across the collector-base junction is fixed at zero, the depletion region width remains constant regardless of the current flowing through the emitter. This effectively eliminates the Early effect, making the diode-connected PNP an exceptionally stable voltage reference compared to a transistor operating in the standard forward-active region.
Can I use a diode-connected PNP to drop voltage in a power supply line?
Technically, it will drop about 0.65V, but it is a terrible choice for power supply voltage dropping. The power dissipation limit of a small-signal TO-92 package is around 625mW. At a 0.65V drop, you can only pass about 1 ampere before exceeding the thermal limits of the package (and practically, the tiny wire bonds inside will fail long before that). Furthermore, the Vbe drop is not tightly regulated; it will sag as current increases and drop further as the part heats up due to its -2mV/°C temperature coefficient. Use a dedicated linear regulator or a power Schottky diode for voltage dropping tasks.
