Active Devices › Gain Control
AGC / VGA Calculator
Design automatic gain control (AGC) and variable gain amplifier (VGA) systems. Computes gain control range, control law (linear/logarithmic), NF vs gain trade-off, dynamic range, settling time and control voltage requirements.
AGC System Specifications
dB
dB
dB
V
V
dB
dB
MHz
ms
dBm
Design Notes
Dynamic range = Gmax − Gmin
AGC must handle Pin = Pout,target − Gmax to Pout,target − Gmin
Log control law: ΔG/ΔVctrl = constant (dB/V)
→ Same dB step for each volt of control
Lin control law: G ∝ Vctrl
→ Poor resolution at low gains
NF vs gain trade-off:
At low gain, first-stage NF dominates — system NF rises
Fsys = F1 + (F2−1)/G1 + …
With VGA at front: Fsys rises as GVGA decreases
Settling time: determines AGC loop bandwidth
BWloop ≈ 0.35/tsettle
AGC must handle Pin = Pout,target − Gmax to Pout,target − Gmin
Log control law: ΔG/ΔVctrl = constant (dB/V)
→ Same dB step for each volt of control
Lin control law: G ∝ Vctrl
→ Poor resolution at low gains
NF vs gain trade-off:
At low gain, first-stage NF dominates — system NF rises
Fsys = F1 + (F2−1)/G1 + …
With VGA at front: Fsys rises as GVGA decreases
Settling time: determines AGC loop bandwidth
BWloop ≈ 0.35/tsettle
Architecture tip: Place the VGA after the LNA to preserve noise figure at all gain settings. Use multiple gain stages (e.g. IF attenuator + baseband VGA) to cover the required dynamic range without compromising NF.
AGC Design Results
Gain Control
Total gain control range—
Number of gain steps—
Control sensitivity—
Resolution (bits needed)—
Dynamic Range
Min detectable signal Pmin—
Max input signal Pmax—
Input dynamic range—
Noise floor (at Gmax)—
NF vs Gain
NF at Gmax (high gain)—
NF at Gmin (low gain)—
NF change over range—
Loop & Timing
AGC loop BW—
Loop filter capacitor (est.)—
Gain vs Control Voltage
Log law
Linear law
Gain Setting Summary (selected steps)
| Gain (dB) | Vctrl (log) | Vctrl (lin) | NF (est.) | Pin target |
|---|