Question E6E08:
How is power supplied to the most common type of MMIC?  

Choices:  
A: Through a capacitor and RF choke connected to the amplifier input lead  
B: MMICs require no operating bias  
C: Through a resistor and/or RF choke connected to the amplifier output lead  
D: Directly to the bias voltage (Vcc) lead    

Correct Answer is  C: Through a resistor and/or RF choke connected to the amplifier output lead  

To bias the most common MMIC gain blocks (e.g., Mini-Circuits MAR/ERA-series), DC power is fed into the output pin through an isolating element (RF choke and/or resistor), while RF is coupled via capacitors. Thus, the correct answer is: Through a resistor and/or RF choke connected to the amplifier output lead.

Why the correct answer is correct
- Many MMIC gain blocks are two-port devices with no dedicated Vcc pin. The output lead doubles as the DC feed point.
- An RF choke (inductor) passes DC but presents high impedance to RF, keeping RF out of the supply line.
- A series resistor can set the bias current and add RF isolation.
- The RF at the output is then AC-coupled to the next stage via a capacitor.

Why the other choices are incorrect
- A: Through a capacitor and RF choke connected to the amplifier input lead  
  The input is typically DC-grounded internally or not designed for bias. Also, a series capacitor blocks DC, so it cannot supply bias.
- B: MMICs require no operating bias  
  False. MMICs are active devices (GaAs FET, HBT, etc.) and require DC bias for gain and linearity.
- D: Directly to the bias voltage (Vcc) lead  
  The common gain-block MMIC has no separate Vcc pin; bias must be applied via the output lead with isolation. Even when a device has a Vcc pin, you do not connect it “directly” at RF—you use an RF choke and decoupling.

Useful formulas and rules of thumb
- Choose the RF choke so its reactance is high at the operating frequency:  
  $$X_L = 2\pi f L \quad \Rightarrow \quad X_L \gtrsim 5\text{–}10 \times Z_0 \text{ (often }Z_0=50\,\Omega\text{)}$$
- AC-coupling capacitors (input/output) should have low reactance across the band:  
  $$X_C = \frac{1}{2\pi f C} \quad \Rightarrow \quad X_C \ll Z_0 \text{ (e.g., } \le 0.1\,Z_0\text{)}$$
- If a bias resistor is used to set current:  
  $$R \approx \frac{V_{CC} - V_{\text{device}}}{I_{\text{bias}}}, \quad P_R = I_{\text{bias}}^2 R$$
- Typical MMIC gain blocks operate around 3–6 V and tens of mA; always consult the datasheet for the recommended bias network.