Use of Heterojunction Bipolar Transistor technology has resulted in development of super wide-band monolithic microwave amplifiers such as Mini-Circuits ERA series. These amplifiers cover a bandwidth from DC to 8 GHz. They need biasing current injected at the RF output port. As the RF and DC share this port, an inadequately designed DC biasing circuit will degrade the RF performance. It is commonly recommended to use a resistor and RF choke in series with the DC supply. The purpose of the RF choke is to minimize the RF loss caused by the resistor.

Mini-Circuits has developed a super wide-band RF choke covering 50 to 8000 MHz, which will be available as Model ADCH-80A. This paper will describe the results of using this RF choke in biasing wide-band amplifier circuits. Performance characteristics such as gain, return loss, IP3, and power output will be presented and compared against performance measured in a test fixture, in which biasing current for the amplifier is supplied through the bias-tee which is internal to the s-parameter test set of the network analyzer.

How a Super Wide-band RF Choke Maximizes Amplifier Performance

Figure 1 shows the biasing schematic of an ERA amplifier. The biasing resistor is designated by R_bias. Its value is determined by the device voltage, the supply voltage, and the desired operating current of the amplifier. For example, for ERA-1 the device voltage is 3.6V. Assuming a supply voltage of 12V, the biasing resistor is given by:

R _bias = (V _cc - V _d)/I _d = 210 ohms

where V _ccis the supply voltage and I _dis the biasing current. Use of a 210-ohm bias resistor without the RF choke in series will result in 1-dB loss of gain and power output. If the supply voltage is dropped to 5V, then R _bias will be 35 ohms. This will result in a loss of 3.8 dB in gain and power output, and degradation in return loss. An RF choke in series with the bias resistor will add an inductive reactance to the shunt impedance, and minimizes the effect of the resistor on these performance characteristics. This paper presents the performance of the wide-band RF choke, including the improvement it provides in the wide-band amplifier application.

Fig. 1 Biasing Schematic of ERA amplifier

Description of the RF choke

Commercially available inductors can be used as RF chokes. The low end of the frequency range of usefulness is decided by the value of the inductance; the higher the value, the lower the frequency. The high end of the frequency range is determined by the series resonant frequency of the inductor; it tends to decrease as the value of the inductance increases. Thus, there is clearly a limitation on the bandwidth of usage. Besides this, inductors are not clearly specified for RF choke applications. This complicates the circuit designer’s job. Any design changes performed by the inductor manufacturer will have an unknown effect on the circuit.

Fig. 2 Photograph of ADCH-80A

Mini Circuits has designed its super wide-band RF choke to optimize performance over a wide band. Figure 2 is a photograph of the unit and Table-I lists its specifications. It is specified to operate over a frequency range of 50 to 8000 MHz. This is wide enough to cover the ERA-1 amplifier, specified to 8 GHz. The equivalent inductance of the RF choke is one microhenry. For comparison, a typical commercially available one microhenry inductor has a series resonant frequency as low as 90 MHz, which is much lower than that of the RF choke, Model ADCH-80A. Figure 3 shows the schematic of the circuit used to evaluate the RF choke in a 50-ohm system. Figure 4 plots the insertion loss and Figure 5 the return loss at various currents up to 100 mA. Note the insertion loss and VSWR change very little with change in current.

Fig. 3 Evaluation circuits