DC Feed
How a DC Feed Isolates Bias From the Signal Path
Every active RF stage needs two things at once on the same node: DC supply current to set its operating point, and an unobstructed RF path for the signal. The DC feed reconciles these competing demands. It connects the supply rail to the transistor drain or collector through an element that looks like a short circuit at DC (so bias current flows freely) but like an open circuit at RF (so the signal stays on the 50 ohm line instead of dumping into the power supply). The complementary element on the signal trace is the series DC block capacitor, which is transparent to RF but stops the supply voltage from reaching the next stage. Together the feed and block form the bias network around the device.
The simplest feed is a single inductor, the RF choke. Its reactance XL = 2πfL rises with frequency, so a 100 nH choke presents about 628 Ω at 1 GHz, which is high relative to 50 Ω and reflects most of the RF back toward the device. The catch is parasitic capacitance between the windings: every real inductor self-resonates at some frequency where that shunt capacitance cancels the inductance. That same 100 nH part, with roughly 0.25 pF of winding capacitance, self-resonates near 1 GHz; above the self-resonant frequency the choke turns capacitive and its isolation collapses, so the feed only works below SRF. Push toward 10 GHz and a bulk-wound 100 nH choke is useless: you move to a much smaller distributed or conical choke, or to a quarter-wave line, to keep usable isolation. Wideband feeds use conical chokes or cascade several inductor values to push isolation across many octaves.
On planar circuits and MMICs, designers often replace the lumped choke with a quarter-wave bias line. A high-impedance microstrip line one quarter wavelength long, terminated at the supply end in an RF short to ground (a shunt bypass capacitor), transforms that short into a very high impedance at the device end. This needs no inductor and is repeatable in fabrication, but it is inherently narrowband because the quarter-wave condition only holds near the design frequency.
DC Feed Design Equations
XL = 2π × f × L ≥ 10 × Z0
Choke self-resonant frequency (parasitic C limit):
fSRF = 1 / (2π × √(L × Cpar))
Quarter-wave bias line length:
ℓλ/4 = c / (4 × f0 × √εeff)
IR drop across feed (current rating check):
Vdrop = IDC × DCR
Example: a 100 nH choke with Cpar ≈ 0.25 pF self-resonates near 1 GHz, so it is only usable below that, where it gives 628 Ω at 1 GHz; for a 10 GHz feed it is well above SRF, so use a conical (distributed) choke or a λ/4 line instead. At IDC = 2 A through 0.1 Ω DCR, Vdrop = 0.2 V.
DC Feed Topology Comparison
| Feed Type | Bandwidth | Current Handling | Footprint | RF Isolation | Best Application |
|---|---|---|---|---|---|
| Single RF choke | Narrow (below SRF) | Up to 5 A | Small SMD | Good below fSRF | Single-band PA bias |
| Cascaded chokes | Multi-octave | 1 to 3 A | Moderate | Wide, with ripple | Broadband modules |
| Conical (wideband) choke | Decades (1-40 GHz) | 0.3 to 1 A | Vertical, bulky | Excellent, flat | Lab feeds, mmWave |
| Quarter-wave line | ~15% around f0 | Trace-limited | Planar, no parts | Excellent near f0 | MMIC, microstrip LNA |
| Bias tee | Wide (10 MHz-40 GHz) | 0.5 to 2 A | Connectorized | Excellent, specified | Test, active antennas |
Frequently Asked Questions
Why does an RF choke used as a DC feed need self-resonance above the operating band?
A choke is a high impedance only below its self-resonant frequency (SRF), where inter-winding capacitance resonates with the inductance. Below SRF it is inductive and blocks RF; above SRF it goes capacitive and its impedance collapses, letting RF leak into the supply. Pick a choke whose SRF is above your highest frequency, or cascade two values about a decade apart (for example 100 nH plus 10 nH) so one still blocks while the other passes through resonance. Conical chokes spread the SRF across decades for 1 to 40 GHz feeds.
How does a quarter-wave bias line inject DC without an inductor?
A λ/4 line terminated in an RF short (a shunt bypass capacitor to ground) transforms that short into an open circuit at its input, so it blocks RF at the device node while still conducting DC through the copper. It needs no lumped part and is repeatable on MMICs and microstrip, but it is narrowband: the high impedance only holds near f0 and its odd harmonics. A 90 to 110 Ω line plus a radial stub on the bypass end widens the usable band to roughly 15 percent.
What current rating and DC resistance should a DC feed choke have for a power amplifier?
The choke must carry full quiescent plus RF-modulated drain current without saturating or overheating. A GaN PA drawing 2 A quiescent needs a feed rated for at least 3 A continuous, and low DCR so the IR drop stays small: 2 A through 0.1 Ω loses 0.2 V. Saturation current must exceed the peak envelope current, which in Class AB can reach two to three times the quiescent value, so air-core or conical chokes are preferred over ferrite at RF.