Bipolar Junction Transistor Amplifiers:Wideband Amplifiers.

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Wideband Amplifiers

The development of wideband amplifiers with discrete circuits progressed rapidly during World War II with vacuum tubes and into the 1960s with BJT circuits. Although many important techniques were perfected between 1940 and 1960, these methods are not generally applicable to wideband IC design. Many of these approaches to wideband amplifier design required relatively large inductors for shunt peaking and relatively large coupling capacitors to AC couple individual stages. While small inductors are now available on IC chips, the range of values limit their usage. Likewise, the limitation on IC capacitor sizes preclude the use of coupling capacitors for IC amplifiers. These difficulties are mitigated in IC design by using additional transistors, however, the benefit of past theoretical developments is then largely unused. Furthermore, the absence of coupling capacitors in IC amplifiers leads to a much stronger interaction between DC and AC design of each stage than is present in discrete design. Two of the more popular methods of IC wideband amplifier design that overcome the limitations on IC circuits are based on composite stages or feedback cascades [7,8].

Composite or Compound Stages

The wideband amplifier of Figure 23.24 is a classic architecture on which many present amplifiers are based. This schematic is for the RCA CA3040 [8]. The input is a buffered, differential, cascode pair. The transistors T1 and T4 are emitter follower stages, while the pair of devices T2 and T3 forms a cascode stage as also does the pair of devices T5 and T6. Transistor T9 with its emitter degeneration resistance forms a high output impedance current source. Transistors T7 and T8 buffer the output signals to allow relatively high output currents.

Since the input is differential, the input signal can be referenced to ground eliminating the need for a coupling capacitor. The frequency response of the gain extends from DC up to ~55 MHz with a constant gain of 30 dB.

The Motorola MC1490 [8] is another typical wideband amplifier chip that can be used for radio frequency or audio applications. It has a power gain of 50 dB at 10 MHz and 35 dB at 100 MHz. It also has a built-in automatic gain control to allow usage as an intermediate frequency amplifier.

Feedback Cascades

The circuit of Figure 23.25 shows the schematic of the MC1553 wideband amplifier using a feedback triple [7]. Transistors T1, T2, and T3 make up the feedback triple using resistor RF to create a feedback path. This path establishes the incremental voltage gain and also provides DC feedback to keep T1, T2, and T3 in their active regions. There is a second feedback path from the current mirror output transistor

Bipolar Junction Transistor Amplifiers-0229

T6, but this is primarily DC feedback. The capacitor CB is a large, external bypass capacitor that must be added to the circuit to decouple the AC feedback to the input stage. This IC amplifier has a voltage gain of 50 V/V with a bandwidth of 50 MHz.

Although the capacitor CB is required in some applications, if the amplifier is driven by a low impedance source, it is unnecessary to add this element.

In recent years, both wide- and narrowband amplifiers in the 1–6 GHz range have become more significant. Many companies that provide amplifiers in this frequency range use HBT designs with SiGe, GaAs, GaN, or other material to achieve the necessary frequency performance.

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