A wide-band low noise L-band balanced transistor amplifier

The design principles, construction details, and experimental results of a balanced transistor amplifier suitable for precise wide-band applications in the low microwave frequency range are described. Each amplifier stage consists of a pair of electrically similar transistors whose input and output signals are combined by 3-dB directional couplers. This eliminates the need for tuning adjustments. For a wide frequency range, the main advantages of the balanced design over more conventional multistage amplifiers are 1) improved input and output impedance matching, gain flatness, phase linearity, gain compression, and intermodulation characteristics, 2) possible designing of the amplifier simultaneously for minimum noise figure and good input match, 3) relatively little effect on overall amplifier gain and matching by changes in the distribution of transistor impedance characteristics, provided that transistors can be selected in similar pairs. The amplifier gain is easily controlled over a wide range by the dc bias with little degradation of the gain flatness and impedance matches. To obtain these advantages, a second transistor is required for each amplifier stage. A four-stage balanced amplifier was designed and constructed in printed circuit form for L-band operation, using Western Electric GF-40037 transistors in the common emitter configuration. The transistors were paired in a test circuit so that their input or output impedances were within about ten per cent. (With the current GF-40037 distribution, this takes less than five random tries on the average.) Without any tuning adjustments, the following characteristics were obtained over a 20 per cent (and 60 per cent) frequency band in the 0.8 to 1.6-Gc/s range: Gain = 20 dB ± 0.2 dB (±0.5 dB) Reverse Loss >50 dB (>40 dB) Phase < ±1° from linear (±6°) VSWR In < 1.10 (<1.2) VSWR Out < 1.10 (<1.2) Noise Figure≅6 dB Gain Compression ≤ 0.25 dB at 0-dBm output signal level Third-Order Intermodulation- With -3-dBm output power from two signals at frequencies f
A
and f
B
, power out at 2f
A
- f
B
and 2f
B
- f
A
≃ -50 dBm. The 3-dB bandwidth points are at 650 Mc and 1700 Mc, and the voltage standing-wave ratios (VSWR's) over this range are under 2. The transistors are operated at I
e
= 2 to 4 mA and V
CB
= 5 to 6 volts, depending on the stage. The total dc power consumption for the four-stage amplifier is about 50 mA at 7.5 volts.