Bipolar RF Transistors Information
Bipolar RF transistors are designed to handle high power radio frequency (RF) signals in amplifiers, transmitters, monitors, and other devices. Like all transistors, they are used to switch and amplify signals in digital and analog circuits.
Bipolar transistors, also referred to as bipolar junction transistors (BJT), are produced by stacking three layers of semiconductor material. The two different types of material, both of which are capable of conducting current, are referred to as n- and p-type materials. N-type semiconductors operate using excess free electrons, while p-types use excess "holes." In this context a hole is simply the lack of an electron where one could theoretically exist. By stacking layers of p- and n-type materials, semiconductors form multiple p-n junctions, which can be utilized for their various electrical properties.
Based on the principles above, bipolar transistors can be one of two types: NPN (a p-region sandwiched between two n-regions) or a PNP (the opposite of an NPN). The current and voltage polarities of an NPN transistor are opposite those of PNP and vice versa. NPNs are the most widely used bipolar transistors and will be discussed in detail. Each layer or region is connected to a different terminal; the layers and terminals are designated as base (B), emitter (E), and collector (C). The image below shows the cross-section of a basic NPN transistor, complete with labeled terminals.
Image credit: Inductiveload
A primary use for bipolar transistors is to control the output collector current (IC) by varying the current ("current-controlled") or voltage ("voltage-controlled") of the base terminal. Because a very small current or voltage at the base terminal can result in a much larger collector output, bipolar transistors often function as current amplifiers. Current gain represents the factor of current amplification is an important specification relative to this function. While manufacturers typically specify this value, it can also be calculated using the following formula:
β = current gain factor
IC = collector current
IB = base current
A bipolar transistor's ability to amplify signals is based on its n- and p-doped regions. In basic terms, when a positive bias (charge or voltage) is applied to the base terminal, it forces the holes inherent in the p-region into toward the emitter terminal and allows excited electrons to be swept from the n-type emitter region to the collector terminal, resulting in a relatively high current output. This action is shown in the image below.
Image credit: Inductiveload
When a transistor operates in this manner,it is said to be in an active state. Bipolar transistors can also function in two other states:
- Cut-off: the device has no output.
- Saturated: the collector output has reached its maximum value. Any increases in the base terminal's current or voltage will not affect the device's output.
When transistors are used in circuits, one of the terminals must be grounded. Based on this fact, there are three possible transistor configurations, all of which are shown in the table below.
Grounded base terminal; emitter input and collector output.
Grounded emitter; base is input and collector is output. Best current gain capabilities of all configurations.
Collector is common to both emitter and base; base is input, emitter is output and load driver.
Image credits: Dnatechindia | Omegatron
Bipolar Transistors vs. Field-Effect Transistors
Bipolar transistors are often compared with field-effect transistors (FET); each of these devices have inherent advantages and differences.
- BJTs are typically current-controlled, vs. voltage-controlled FETs
- BJTs are capable of higher gain, while FETs have higher impedances
- BJTs typically consume more power
- BJTs are less sensitive to static than FETs
- FETs are unipolar, while BJTs are bipolar
Transistors are the building block of all major electronics and are used in an enormous variety of applications and industries. Bipolar RF transistors are useful as switches and amplifiers in any device that uses radio frequency (RF) signals. Examples of these devices include:
- RF amplifiers
- Radios and other communication equipment
- Cellular phones
- Broadcast equipment
- Avionics equipment
As mentioned above, a transistor's collector output current (IC) is typically specified by the manufacturer and is one of the most important numerical specifications. Other specifications include breakdown voltage, output power, and power dissipation.
- Breakdown voltage is the maximum voltage a device can sustain without physical damage or destruction. Buyers of bipolar RF transistors may consider the collector-emitter breakdown voltage (VCEO) and the collector-base breakdown voltage (VCBO).
- Output power represents the total power produced by the transistor, measured in watts (W).
- Power dissipation is the total power consumed by the transistor, expressed in watts or milliwatts (mW). Because bipolar transistors are easily destroyed by heat, it is extremely important to note how much power a device can dissipate without sustaining damage.
Numerical specifications unique to radio frequency (RF) devices include operating frequency (in Hz, kHz, or MHz), unity gain bandwidth, and noise figure (NF).
Unity gain bandwidth refers to the frequency at which an RF transistor's current gain is unity.
Noise figure (NF) is the ratio, in decibels (dB), of two other ratios: the signal-to-noise ratio at the input and the signal-to-noise ratio at the output. NF is basically expressed as the amount of noise added to a signal during normal operation. Lower noise figures result in better device performance.
The manufacturing and testing of bipolar RF transistors is covered by numerous standards. Common standards include:
BS EN 606747-16 - Microwave ICs: amplifiers
MIL 19500/27 - High frequency PNP transistors