Cascade MMIC amplifier circuit diagram

MMIC devices can be cascaded to achieve greater gain than when used individually, although some rules must be followed. For example, we must realize that MICs have varying amounts of gain in the frequency range from near DC to the microwave region. As with all cascaded amplifiers, we must prevent feedback between different stages. Two factors must be noted here. First, as usual, pay attention to structural layout. The input and output circuits external to the MIC must be physically isolated to prevent coupling feedback. Second, there must be two or more levels of decoupled DC power lines. Signals on DC power lines can easily couple between different stages, causing unwanted feedback.

Figure shows the method of decoupling the DC supply lines in a two-stage MIC amplifier. In a low-frequency cascade amplifier, the V+ terminals of resistors F1 and R2 can normally be connected together and connected to the DC power supply. Decoupling between different stages can be accomplished by just one capacitor at the connection point. However, as the operating frequency increases, the situation becomes more and more complicated, partly because the actual device is not very ideal. For example, in an audio amplifier, electrolytic capacitors used in power supply ripple filtering are sufficient for decoupling. However, at RF frequencies, electrolytic capacitors can only act as capacitors (they act more like resistors at these frequencies).

The decoupling system in the figure includes a circuit of radio frequency chokes RFC3 and RFC4 and capacitors C4 to C9. RF chokes are used to block high frequency AC signals from passing through power lines. These chokes have extremely high reactance at very high frequencies (VHF), but very low resistance at DC. For example, a 1μH choke may have only a few ohms of DC resistance. But (according to 2πFL) there is an impedance of 3000Ω at 500MHz. It is important for RFC3 and RFC to be placed as shown to reduce the mutual inductance due to their magnetic fields.

Figure cascade MIC amplifier circuit diagram