What Is a Diode Connected Transistor and Why Use One?
A diode connected transistor is a bipolar junction transistor (BJT) or MOSFET deliberately wired to force it into a specific operating region, effectively turning it into a two-terminal diode. For an NPN BJT, you achieve this by shorting the base pin directly to the collector pin. For an N-channel MOSFET, you short the gate to the drain. The direct answer to why we do this instead of using a standard 1N4148 signal diode comes down to one word: matching.
When you look at the physical pinout of a standard TO-92 NPN transistor like the 2N3904, the pins (viewed from the bottom with the flat edge facing you) are Emitter (E), Base (B), and Collector (C). To create the diode connection, you wire your circuit to the Emitter and the Base, while physically jumpering the Base and Collector together. In schematic symbols, this is drawn as a standard transistor symbol with a wire looping from the base node to the collector node.
Operation Regions and Biasing Basics
When you short the base to the collector on an NPN BJT, the base-collector voltage (Vbc = 0V). Because the collector-base junction is never reverse-biased, the transistor cannot enter deep saturation. It is forced to operate right on the edge of the forward-active region. For a MOSFET, shorting gate to drain ensures Vds = Vgs, which inherently satisfies the condition for the saturation (active) region (Vds > Vgs - Vth).
Here is how the operating regions and typical voltage drops break down across common device types:
| Device Type | Connection | Operating Region | Typical V_drop | Typical I_range |
|---|---|---|---|---|
| NPN BJT (e.g., 2N3904) | Base to Collector | Forward Active (Edge) | ~0.60V to 0.70V | 10µA to 50mA |
| PNP BJT (e.g., 2N3906) | Base to Collector | Forward Active (Edge) | ~-0.60V to -0.70V | -10µA to -50mA |
| NMOS (e.g., 2N7000) | Gate to Drain | Saturation (Active) | Vth + Vov (~2.0V) | 1mA to 200mA |
| PMOS (e.g., BS250) | Gate to Drain | Saturation (Active) | ~-2.0V to -3.5V | -1mA to -150mA |
How to bias and select it: A diode connected transistor is not a power rectifier; it cannot handle high surge currents. You must always limit the current using a series resistor or a dedicated current source. When selecting a part, ensure its maximum collector current (Ic) and power dissipation (Pd) ratings exceed your bias current by at least 50%. For low-current precision biasing (under 5mA), small-signal BJTs are ideal. For higher current active loads, use power MOSFETs.
The Current Mirror: A Complete Application Circuit
The most common application for this configuration is the BJT current mirror. Let's build a basic mirror to sink a precise 5mA to drive an indicator LED, using standard 5V logic.
Components Required:
- Q1: 2N3904 (Diode connected reference)
- Q2: 2N3904 (Output mirror transistor)
- R1: Bias resistor (calculated below)
- Vcc: 5.0V DC supply
The Math:
We want the reference current (I_ref) through Q1 to be 5mA. The voltage drop across Q1's base-emitter junction (Vbe) will be approximately 0.65V at this current. The voltage across R1 is therefore 5.0V - 0.65V = 4.35V.
Using Ohm's Law: R1 = 4.35V / 0.005A = 870Ω. The closest standard 1% resistor is 866Ω, or we can use a standard 5% value of 910Ω (which yields ~4.78mA). Let's use 910Ω for easy sourcing.
- Wire the Reference Leg: Connect one end of the 910Ω resistor (R1) to the 5V Vcc rail. Connect the other end of R1 to the Base and Collector pins of Q1 (jumper them together).
- Ground the Reference: Connect the Emitter pin of Q1 directly to circuit ground.
- Wire the Mirror Leg: Connect the Base pin of Q2 directly to the Base/Collector node of Q1. Connect the Emitter of Q2 to ground.
- Connect the Load: Connect your LED (with an appropriate series dropping resistor if needed, though the current is already limited to ~4.8mA) between the 5V rail and the Collector pin of Q2.
- Power and Verify: Apply 5V. Q1 will drop ~0.65V, biasing Q2's base. Q2 will sink the same ~4.8mA through the LED, mirroring Q1's current regardless of minor supply voltage fluctuations.
Bench Scenario: Building a Temperature-Compensated Bias Network
Theory is clean; the workbench is messy. Here is a real-world scenario demonstrating why thermal coupling matters when using a diode connected transistor for biasing.
The Setup: I was repairing a vintage Class AB push-pull audio amplifier. The output stage used two TO-3 power transistors (NPN and PNP) mounted on a large finned heatsink. The original bias circuit used a single diode connected transistor to drop roughly 0.65V, providing the turn-on bias voltage between the bases of the output pair to eliminate crossover distortion.
The Numbers: At room temperature (22°C), I set the idle bias current to a safe 25mA per output device. The diode connected transistor was dropping exactly 0.64V. Everything sounded great on the bench.
The Outcome and What Went Wrong: I buttoned up the chassis and ran a 4-ohm dummy load at 20W for thirty minutes. The output transistors heated up to about 65°C. Silicon junctions have a negative temperature coefficient of approximately -2mV/°C. As the output transistors heated, their required Vbe to maintain 25mA dropped from 0.64V down to roughly 0.55V.
However, the diode connected bias transistor was mounted on the main PCB, three inches away from the heatsink. It stayed at 30°C, still dropping 0.63V. The bias voltage was now nearly 100mV too high for the hot output devices. The idle current spiked to 400mA, which generated more heat, which dropped the Vbe further, which spiked the current more. This is classic thermal runaway. Within two minutes, the idle current hit 2.5A, the bias transistor popped, and the main 5A fuse blew.
The Fix: You cannot just float a bias transistor on the main board. I replaced the single diode connected transistor with a proper Vbe multiplier circuit (a transistor with a voltage divider across base and collector) and physically bolted it directly to the output heatsink using thermal paste and a mica insulator. As the heatsink warmed, the bias transistor warmed equally, dropping its voltage in perfect lockstep with the output devices. Idle current remained rock solid at 25mA.
Safe Default Part Numbers and Multimeter Testing
When you are prototyping on a breadboard and need a diode connected transistor, reach for these safe, cheap, and widely available defaults:
- 2N3904 (NPN BJT): 40V Vce, 200mA Ic, ~$0.05. The undisputed king of low-side small-signal biasing.
- 2N3906 (PNP BJT): -40V Vce, -200mA Ic, ~$0.05. Use for high-side current mirrors.
- 2N7000 (NMOS): 60V Vds, 200mA Id, ~$0.10. Great when you need a higher voltage drop (Vth ~2V) or zero gate leakage current.
- BCM847 (Matched NPN Pair): 45V, 100mA, ~$0.60. A dual transistor in a single SOT-23-6 package. Because both dies are in one package, they are thermally coupled and electrically matched from the factory. Ideal for precision mirrors without buying expensive legacy parts like the MAT04.
How it fails: The most common failure mode is overcurrent melting the base-collector short junction, turning the device into an open circuit. In thermal runaway scenarios, the junction overheats and shorts internally, dropping the voltage to near zero and destroying downstream components.
How to test it with a multimeter:
- Set your digital multimeter (DMM) to Diode Test mode (the symbol with an arrow and a line).
- Identify the pins. For a 2N3904, the flat side facing you means pins are E, B, C.
- Place the Red probe on the Base pin (which is internally shorted to the Collector in your circuit).
- Place the Black probe on the Emitter pin.
- The DMM should display a forward voltage drop between 0.600V and 0.700V. If it reads OL (open), the junction is blown. If it reads near 0.00V, the junction is shorted.
- Swap the probes (Black to Base, Red to Emitter). The meter should read OL (reverse biased). If it reads a voltage, the transistor is leaky and must be binned.
By understanding the exact operating region, respecting the thermal coefficient, and using matched pairs where precision matters, the diode connected transistor transitions from a textbook oddity to one of the most reliable biasing tools in your analog design toolkit. For deeper mathematical modeling of the Early effect in these configurations, refer to the MIT OpenCourseWare Integrated Analog Electronics materials.
