Class B Bias Point
Understanding the Class B Bias Point
The Class B Bias Point defines the precise DC electrical conditions required to operate a transistor in a Class B mode. The defining characteristic of this bias point is absolute zero quiescent current (Idq = 0 A). When the transmitter is standing by and no RF signal is applied to the input, the power supply delivers zero current to the device, resulting in zero heat dissipation and zero battery drain.
Achieving this requires applying a DC gate (or base) voltage that sits exactly on the knife's edge of the transistor's turn-on threshold (Vth or Vpinch-off). In a depletion-mode device (like a native GaN HEMT), the gate must be biased with a strong negative voltage (e.g., -4.0V) to hold the channel fully closed. In an enhancement-mode device (like an LDMOS or bipolar transistor), the bias sits near 0V or just slightly below the 0.7V turn-on threshold. At this exact point, the device acts as a perfect half-wave rectifier: the slightest positive RF swing turns the transistor on, and the slightest negative swing holds it off.
Challenges at Microwave Frequencies
While the mathematical theory of the Class B bias point is elegant, it is incredibly unstable in physical microwave hardware. Transistor threshold voltages drift significantly with temperature. If the device heats up and the threshold voltage drops, an amplifier biased exactly at Class B will accidentally drift into Class AB (drawing current) or Class C (cutting off too much of the signal). Precise, temperature-compensated bias tracking circuits are mandatory to lock a transistor to a true Class B bias point.
Idle State: Idq = 0 Amps → PDC_idle = 0 Watts
Conduction Rule:
If VRF_in > 0 : Iout > 0 (Conducting)
If VRF_in ≤ 0 : Iout = 0 (Pinched Off)
Comparison
| Transistor Tech | Typical Vthreshold | Class B Bias Point Action |
|---|---|---|
| Bipolar (BJT) | +0.7 V | Bias base exactly at +0.65V to +0.7V. |
| Silicon LDMOS | +2.0 V | Bias gate exactly at +2.0V. |
| D-Mode GaN HEMT | -3.5 V | Apply -3.5V to gate to pinch off channel. |
Frequently Asked Questions
Why is it so hard to maintain a Class B bias point?
Because the threshold voltage (Vth) of semiconductors is highly temperature-dependent. As an LDMOS transistor heats up during transmission, its threshold voltage drops. If you fed it a constant 2.0V bias, it might start at Class B when cold, but quickly drift into a highly-conductive Class A state as it gets hot, leading to thermal runaway and destruction.
How do active bias controllers work?
Active bias controllers use an external operational amplifier and a temperature sensor (often a diode mounted on the same flange as the RF transistor) to actively monitor the temperature. As the RF transistor heats up, the controller instantly adjusts the gate bias voltage downwards to perfectly track the shifting threshold, keeping the amplifier locked at Class B.
Can I use a Class B bias point for an Envelope Tracking (ET) amplifier?
Yes, Class B bias points are highly favored for Envelope Tracking systems. In ET, the drain supply voltage is dynamically altered to track the RF envelope. By keeping the gate biased at Class B, the transistor acts efficiently as an RF current source, allowing the drain modulator to dictate the final output power with incredible overall efficiency.