Amplifier Biasing Made Easy

Jacob Trevithick | April 16, 2020 Amplifier fundamentals Amplifiers boost Power Supply signal strength.

The Basics RF in RF out • Active Device • Amps Require DC bias voltages to operate • Constructed with Bias transistors Setting • External DC power converted to RF signal power

Marki Microwave Inc. © 3 Amplifier Biasing Considerations can be overwhelming

1. Power Supply • Provides Current • Affects Linearity But they don’t +Vd/Vc

have to be! 3. Output -> Supply Volt. Line Feedback Bypass Circuitry Low Freq. Oscillations Power Supply Noise Feedthrough 1. Power Supply 2. Bias Setting 4. Linear and nonlinear Controls bias point RF in + metrics are sensitive on the IV curve for RF out to power supply and your application + bias voltages. Amplified Gate/Base Voltage Noise Heat Dissipated 3. 3. Bypass 4. Reliability through Chip Backside Bypass Circuitry circuitry Transistors can be Noise filtering, damaged by over stability, and bias or improper 2. • Noise Filtering switching time are sequencing Vg/Vb • Protection Against Oscillations considerations • Sets Bias Point • Operating Class Bias • Harmonic Generation Setting Marki Microwave Inc. 4 Transistor type determines bias requirements

Supply Bias Supply Bias

Bias RF output Circuitry RF output

RF input RF input

FET/HEMT

BJT/HBT

FET Amplifiers BJT Amplifiers • Gate current is negligible • Base Current is always required • Gate can be biased with just a • Standard Current mirror typically used resistive network

Marki Microwave Inc. 5 FET Specific Biasing: D-Mode vs. E-mode

Depletion Mode Negative Bias, Requires Sequencing

• Negative threshold voltage S D • Transistors conduct without negative N++ G N++ gate bias • Sequencing typically required to avoid damage

Channel Formed with Vgs = 0V Enhancement -4 -3 -2 -1 0 1 2 Mode Positive Bias, No Sequencing • Positive threshold voltage • Transistors are off without gate bias S D N++ G N++ • Eliminates the need for sequencing and negative voltages

No Channel Formed 3 0.5 1 1.5 2 2.5 with Vgs = 0V Marki Microwave Inc. 6 BJT Amplifier Biasing: Two Main concerns Bias

Supply Current Supply Decreases Increases Thermal Runaway Current Mirror sets RF Transistor • Self Heating can cause Base Voltage thermal runaway

• Ballasting resistor used to dampen this Ballasting feedback loop Junction Temp. Power Resistor RF output Increases Dissipation RF input Increases BJT/HBT

Voltage Sensitivity 푉퐵퐸 • Collector current is 퐼퐶 ∝ 푒 exponential with base BJT Amplifiers Typically voltage. Require Current Mirrors • Circuitry required to • Makes biasing as easy as precise control 푉 퐵퐸 setting a voltage

Marki Microwave Inc. 7 Electrical Performance

Saturated Power Gain Major Harmonic Intercepts

Saturated Output Power (dBm) vs. Frequency, VC = 6 V Effects 1 dB Compression 30 Phase Noise 25

20 Current Consumption

10 VB = 3 V VB = 4 V 5 VB = 5 V VB = 6 V 0 0 5 10 15 20 25 30 35 Frequency (GHz)

Return Losses Minor Isolation Effects Max safe operating conditions

Marki Microwave Inc. 8 Nonideal bias components On Chip vs. Off Chip Biasing

Bypass Capacitors • On chip bypass capacitors are limited because of size and dielectric.

Current Handling Inductance Bypass vs. Blocking Caps • Thin printed metal lines are • With thin wires and no • Blocking capacitors block DC limited to couple hundred magnetic material, on chip voltages and pass RF signals inductors are limited to >1 GHz. mA of current handling. • Bypass capacitors bypass stray RF signals to ground

Marki Microwave Inc. 9 Biasing affects the amplifier’s reliability

Heat Flow Current Flow

S D S D S G G G G Quiescent vs. Saturation • Higher input Substrate power typically draws more current

Heating Max Power • During operation, • Exceeding max power is dissipated as input power specs heat in transistor will cause damage junctions to transistors

Cooler amps perform better and last longer

Marki Microwave Inc. Courtesy of David Wang, Global Communication Semiconductor 10 Please take a moment to answer a three short questions

1. What amplifier 2. What are some specifications are limitations of on- 3. What is the best majorly affected by chip bias way to filter out bias condition? components? power supply noise?

Output Power Lack of magnetic Bypass circuitry material Return Losses Input and output Current handling DC blocks Harmonic generation Size restraints Sequencing Gain All of the Above Current Mirrors Bias Generation: Bypassing

Tradeoff: Switching Time vs. Low Freq. Noise filtering Supply LC Resonances -> Bias Supply Possible Oscillations

RF output

RF input

Switching Time Immunity to Oscillation • Additional bypass capacitors • Pay careful attention to LC resonant will increase noise filtering frequencies • Use 5ohm -20ohm resistors to de-Q • Increases charge constant on large bypass capacitors input/output, increases • Off-chip shunt caps and series inductors switching times limit feedback oscillations for multistage amplifiers Marki Microwave Inc. 12 Bias Generation: Multistage Amps

Vd (2 V to 4 V) 10 Vd Vd Vd Ohms

200pF 0.1 uF

RF Output … 50Ω trace Vd1 Vd2 Vd3 Vd4 RF input RF In 50Ω

RF Out Power Divider – Inverter Balun 50Ω Vg1 Vg2 Vg3 Vg4 50Ω trace Vg Vg Vg

10 0.1 uF 200pF Ohms Vg (-0.4 V to -0.6 V) Multi-Stage Amplifiers Application circuit • Modifying interstage biases for • Drain supply need to be individually gain/Psat/efficiency? broken out • Gate bias pads resistively connected on- chip Marki Microwave Inc. 13 Bias Generation: Incorrect Supply/Bias Voltage? Supply +12 V Step Diodes for Power Supplies Bias • Diode voltage drop is -5 V 1V Step Diode (x5) relatively constant with 9 kΩ Supply current change • Size step diodes for max 1 kΩ Bias current handling

-0.5 V

RF output +7 V RF output

Resistive Dividers for RF input RF input Bias Voltages

• Bias current is low and FET/HEMT constant FET/HEMT • Size resistors based on gate/base current draw

Marki Microwave Inc. 14 Bias Generation: Negative Bias

Voltage Inversion and Sequencing • Apply +5V and produces sequenced supply and negative bias voltages • Negative Voltage can be produced by Charge inverter chip • Sequencing is available with COTS parts • Minimal cost, larger board size

Introducing Marki Microwave’s New UC5 Single-Supply Voltage Sequencer Package for AMM Amplifiers

Marki Microwave Inc. 15 Marki Amplifier Catalog

Positive Only Low No Sequencing/ mm-wave LO driver Amplifiers Phase Noise Amps Grounded Gate Medium Power for mm-wave LO High power for high Optional Amps Sequencing/Negative bias circuitry available linearity Medium power square High gain High gain wave LO driver Surface mount, bare die, and connectorized Surface mount, bare amplifiers for general module die, and connectorized purpose mixer driving module For More Information Tech Notes Videos [email protected]

Empower our customers to design faster, simplify production, eliminate complexity, and shatter performance barriers Marki Microwave Inc. © 16 April 30th : A Brief Guide to Mixer Spurs Presented by: Harley Berman & Christopher Marki

May 14th : High Frequency Packaging from the Experts Presented by: Christopher Marki

Registration links and a recording of this webinar will be provided through email. Thank you for joining us today! Bias Generation: Temp Compensation

Small Signal Gain (dB) Over Temperature vs. Frequency, 5V/5V Bias 20 Amplifiers Degrade with 18 16 Temperature 14 12 Increasing bias voltage with increasing 10 8 temperature can counteract gain/power -40 C 0 C 6 25 C 65 C 4 85 C 100 C degradation 125 C 2 Compensation can be digital or analog 0 0 5 10 15 20 25 30 35 Must be careful about reliability Frequency (GHz)

Temp. ퟒ ퟑ … Sense Adjust Bias to Gain Temperature Correct Gain Change +10 dB 3V/3V 3V/4V …

+11 dB 3V/4V … …

… … … 6V/6V Marki Microwave Inc. 18 BJT Bias Current Mirror Circuitry

Bias RF transistor base voltage setting current mirror

Supply

Current mirror used to control

the Base voltage of the RF transistor

RF output RF input BJT/HBT

Bias RF output Circuitry RF output

RF input RF input

FET/HEMT BJT/HBT

Ohmic Linear Saturation Active

Quiescent operating point Quiescent operating point

Load-Line Load-Line

Cut-off Cut-off