Amplifier Design in Software

Creator Code 128B in Software Amplifier Design

frequencies (without encountering any series resonances), and a high inductance coil used to block the lower frequencies The use of two chokes can be necessary because the low frequency, high impedance inductor will begin to pass the high RF frequencies through the natural turn-to-turn capacitance inherent in any coil, but it will do so far earlier than the smaller, minimal parasitic RF choke To minimize the lower-frequency inductor s (L2) significant parasitics from adversely affecting the MMIC s output, the first choke up from the amplifier stage should always be the RF inductor (L1), followed by L2 An additional small value RF bypass capacitor to ground can be placed between L1 and L2 to further decouple any RF from the power supply This type of wideband decoupling will permit the amplifier s complete passband to enjoy a nearly flat gain response over its entire frequency range

There are various coupling techniques that can be used between stages, depending on frequency, cost, performance, and impedance matching needs Capacitor coupling (Fig 3131), also referred to as RC coupling, is found in AC and RF amplifiers only, and is capable of amplifying over a very wide bandwidth (the amplifier s required impedance matching circuit will limit this bandwidth, however) As shown in the figure, the series coupling capacitor CC blocks the DC bias to the next stage, but allows the RF signal to pass through unattenuated CC and R6 form a voltage divider, allowing most of the RF signal to be dropped across the high resistance of R6 located at the input to the next stage The voltage divider functions as described because the capacitor has a much lower impedance to the RF than does the resistor This signal across R6 will then add to or subtract from the second stage s emitter-base junction, forcing its collector current to vary through R7, producing an amplified output voltage RC coupling is a simple method to transfer energy from one circuit to another, but has great difficulty matching the stage s impedances and, unless we employ a low-value

series resonant coupling capacitor at the stage s input and output, this coupling method does not help to attenuate harmonics from being transferred from stage to stage, and from being further amplified The small, series resonant coupling capacitor will also assist in stabilizing the amplifier chain, which it accomplishes by somewhat attenuating the lower and higher (undesired) RF frequencies with its low capacitance value and series resonance operation, while easily passing the frequencies of interest A L, T, or PI network is normally added for harmonic attenuation and impedance matching requirements The coupling capacitor can be part of the matching circuit itself Inductive coupling (Fig 3132), also referred to as impedance coupling, is found in AC and RF circuits, and is comparable to RC coupling However, instead of exploiting a resistor in the collector circuit, it uses a collector inductor Inductive coupling has the advantage in that the collector inductor wastes little DC power due to its very small DC series resistance, thus permitting for more efficient amplifier operation This high-value inductor works as a transistor s collector load because of its high reactance to the alternating collector current, which produces an AC voltage drop This action will subtract from or add to the voltage from the transistor s emitter-collector Inductive coupling is only practical over a relatively narrowband of frequencies, since XL changes directly with frequency, and stage gain would vary as well Direct coupling (Fig 3133), also referred to as DC coupling, is valuable for very low frequency and DC amplification R3 functions as a collector resistor for Q1, and as a base resistor for Q2, and must be carefully chosen to function in both roles, since a small temperature induced current change in Q1 is directly amplified by Q2 Precision components and tight placement of parts to allow each component the same changes in temperature must be used to stabilize this circuit Low-frequency transformer coupling (Fig 3134), employing laminated iron cores, can be adopted for low-frequency AC amplifiers This method, due to the interstage transformer, will not pass the DC bias, and can be used to match the relatively high