The rapid advance in transistor technology into the microwave-frequency region makes this article on microwave transistor trade-offs especially timely. Designers of microwave circuits using transistors should find the information presented here very helpful in their work. It will also bring other readers up to date on a subject of vital importance to the microwave industry.
Part 1 deals with basic considerations and evaluates the effect of dc parameters on microwave-circuit performance.
Part 2 will relate rf parameters to circuit performance and discuss transistor characteristics for power amplifier applications.
Part 3 will cover microwave-transistor thermal effects and VSWR considerations and will interrelate the dc-rf circuit parameters to microwave circuits. E.T.E.
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Part 1: Basic Considerations
Rf power transistors have recently moved rapidly into microwave applications both at sub-harmonic and direct-operating frequencies. The state-of-the-art should continue to advance, especially the maximum operating frequency, reliability, packaging and for transistors tailored to specific design applications. At present, both general purpose and tailored transistors for specific microwave applications are available. It is important, therefore, for the circuit designer to have an insight into the trade-offs involved to better understand the design problems and achievable circuit performance.
For a given transistor type and its processing, there are definite differences in high-frequency performance and interaction with the circuit. These differences can be related to the basic dc and rf parameters of the transistor. Knowing these relationships, it is then possible for the circuit designer to better understand the differences between transistors and for the equipment designer to better tailor transistor parameters to meet specific performance criteria.
The bipolar transistor now offers reliable power outputs up to 50 W at 150 Mc and 15 to 20 W at 400 Mc. The power-frequency state-of-the-art at present is shown in Fig. 1. This is primarily for Class C, cw power output. Most microwave power generation uses Class C amplifiers at lower frequencies with multiplication, or direct Class C amplification at the planned output frequency.
Fig. 1. Power-frequency state-of-the-art for transistors operating cw Class C.
Selecting a transistor
Some basic guidelines for the circuit designer in selecting a transistor are:
- Do not introduce a transistor to a circuit that is not compatible. Understand the characteristics of the semiconductor device before designing circuits around it.
- Exercise caution in package selection. Requiring more than is needed is costly; requiring less than needed causes trouble.
- Selecting a semiconductor device on cost alone becomes a costly mistake if needed performance is sacrificed.
- Select a device which is both available and will do the proper circuit job.
- Stay within the recommended limits set by the manufacturer on its data-sheets. This requires a complete understanding of the data-sheet parameters and their interrelated use.
- Don’t overspecify the semiconductor device. Give the widest possible tolerances to save cost and improve delivery of the device from the vendor.
- Understand maximum device ratings and how to apply them in combination. Not all information on possible combinations of maximum ratings can always be included on the data sheets.
- Evaluate both the device and the circuit operation thoroughly, including all of the stress levels. Stress levels often dictate the semi-conductor device to use. How stress levels can be applied to the transistor requires a complete knowledge of its parameters and capabilities.
- Obtain assistance from the applications engineering department of the semiconductor manufacturer. It is less costly to ask a question early in the circuit design than after all of the devices have been destroyed!