Transistors are often associated with chips, where terms like miniaturization and nanometers are commonly thrown around. This is because they are the basic building blocks in modern electronics, including digital circuits, because they provide amplification and switching.
As discrete components, they are the most common semiconductor devices, with MOSFETs being the most abundantly used. Transistor hFE applies to BJTs, which are their close cousins. To pick the most suitable BJT for a circuit, you must consider this critical parameter, and here’s all you need to know about this property when building amplification circuits.
What is hFE in a Transistor?
Also known as the DC current-gain or forward current transfer ratio, hFE refers to the forward current gain that BJTs can achieve when operating in their active region in circuits. Since most transistors operate as amplifiers when connected in a CE (Common Emitter) configuration, hFE applies to this amplification mode.
The mathematical representation for hFE is IC/IB, which is the ratio of the collector current to base current. So for instance, if the hFE is 50, it means for every unit of base current, the collector current will be 50 times that. If IB is 1mA, then the collector current will be 50mA.
hFE varies significantly due to factors like temperature and the individual transistor’s characteristics, so transistors of the same type and model usually have different DC current gain values.
How hFE Works
To determine the operating principle of hFE, we must look at the internal structure of a BJT, which has three doped semiconductor regions.
Amplification occurs in the active region, which is achieved when the base-emitter junction is forward-biased and the base-collector junction is reverse-biased. The base current controls the operation of the transistor, affecting the flow of the majority charge carriers (electrons or holes) across the junctions.
A large hFE value means the transistor can control a large collector current using a small base current.
How To Find a Transistor’s hFE
The easiest way to find a transistor’s hFE value is by looking at its data sheet, which is provided by the manufacturer. But these datasheets only provide a range for the hFE rather than a specific value.
Manufacturers give this parameter as a range because it varies between transistors of the same type due to manufacturing tolerances and differences, which are difficult to control. It can also vary depending on the operating conditions, such as collector current and temperature.
So to find the specific hFE value, you have to calculate it by applying a known base current into the transistor and measuring the collector current to substitute in the formula. Alternatively, you can use a multimeter to measure a transistor’s hFE directly.
Either way, this DC current gain should only help you to understand how the component works and how it will operate in the circuit. Try to avoid designing amplification circuits that rely heavily on precise hFE values to function because external conditions like temperature can alter it on the fly.
How To Calculate a Transistor’s hFE
NPN and PNP amplification circuits
To get the actual hFE value via calculation, you have to set up a circuit where you control the base current (IB) and measure the collector current (IC).
Measure the Base Current
To measure the base current, connect a known resistance to the transistor’s base terminal and apply a known voltage across it. Calculate the current across this resistor or the base current using Ohm’s law by dividing the applied voltage by the known resistance.
Measure the Collector Current
With the base current identified, measure the collector current to get the values needed to calculate hFE (IC/IB). Connect another known resistance to the collector terminal (in series), then measure the voltage drop across it when the transistor is on. Use Ohm’s law to calculate the collector current.
Calculate the hFE
Use the two values above to calculate the forward current transfer ratio using IC/IB.
How To Measure a Transistor’s hFE Using a Multimeter
Measuring the hFE value using a multimeter is more straightforward because you don’t need resistors. Using common transistors like the 2N2369 or S8050 NPN types, identify the collector, base, and emitter pins using their pinout diagrams, then insert them into the multimeter’s transistor port. Ensure you use the NPN side for these two and PNP if measuring the hFE for PNP transistors.
A digital multimeter with hFE measurement functionality for NPN and PNP transistors
Set the multimeter to the hFE position using the middle selector knob and read the hFE value on the display.
Different Current Gain States
Transistors operate in three regions, and each provides different current gain behaviors.
Active/Linear Region
Transistors operate as amplifiers in this region, where the base-emitter junction is forward-biased and the base-collector junction is reverse-biased. The current gain in this region is the hFE or Beta.
Saturation Region
In this region, both junctions are forward-biased and the transistor is fully on, allowing maximum current to flow through. As the base current increases, the collector current follows linearly until it reaches a saturation point where it can’t rise further.
Cut-Off Region
Transistors operate as off switches in the cut-off region because both junctions are reverse-biased. So the current gain is zero because there’s no base and collector currents.
Factors Influencing Transistor hFE
The forward current transfer ratio in a CE configuration can vary depending on these factors.
- Temperature: As the internal temperature of the transistor increases, its hFE reduces, meaning its amplification properties go down. This occurs due to the increase in concentration of the minority charge carriers, which affect/repel the flow of majority charge carriers.
- Collector Current: hFE generally increases as the collector current rises, but this ratio is not linear. After hitting the peak, hFE starts to decrease.
- Manufacturing Variations: Differences in manufacturing processes and tolerances ultimately make transistors even from the same plant and production line have different hFE values. This occurs due to tiny variations in processes like doping levels, which affect the quantity of minority and majority charge carriers in the BJT. Differences in the base thickness can also alter the charge carrier ratio, affecting hFE.
- Collector-Emitter Voltage: VCE can also affect the DC current gain, although its impact is minimal.
- Signal Frequency: At high switching frequencies, the transit time for charge carriers becomes longer than the signal period, making the transistor incapable of responding fully. This lowers its current gain.
- Aging: Transistors and semiconductors wear out over time, especially when subjected to long periods of use because the high temperatures, voltages, and currents degrade the mechanisms inside. Therefore, the hFE value reduces with time.
The Role of hFE in Transistors
hFE is important in these key areas of a transistor and its electronic circuits.
Signal Amplification
Transistors are primarily used to amplify signals, and hFE shows the amplification capabilities of these components in circuits. The higher the hFE, the better its signal amplification properties.
Transistor Biasing
Biasing schemes often rely on the hFE value to determine the required base current for a given collector current to ensure proper stability and operations at different temperatures.
Circuit Design
In CE configurations, the amplifier’s gain is directly proportional to its hFE value. Therefore, this value can help you determine how to design the circuit, for instance, by helping to calculate the base resistor value.
Transistor Switching
Digital logic circuits employ transistors as switches, and the hFE value helps to calculate the required control signal (base current) to drive the switch into saturation mode.
Other roles include:
- Creating Feedback Networks: To design control systems
- Performance Optimization: Useful for optimizing filters, oscillators, and other analog circuits
Transistor hFE vs. Other Types of Transistor Gains
Besides hFE, transistors provide other types of gains, and here’s how they compare.
hFE or Beta (β)
As stated earlier, hFE is the DC current gain in transistor circuits designed in the CE configuration. The Beta symbol β is used for NPN transistors, while β’ is used for PNP transistors, and generally, the hFE value ranges from 20–1000 for general-purpose transistors.
hfe
In all small letters, hfe refers to the AC current gain, which is specified for a given frequency. This gain is basically the same as hfe but is measured under changing voltage and current conditions.
Alpha (α)
Alpha is the DC current gain measured from a BJT in the common-base configuration. Its value is usually near 1 and its formula is a ratio of the DC collector current (IC) to the DC emitter current (IE), or IC/IE.
Voltage Gain (Av)
The output-to-input voltage ratio is important in some transistor amplifier circuit configurations, and this ratio is the voltage gain or Av.
Power Gain (Ap)
For power applications, such as power amplifiers, the output-to-input power ratio is also important, and this ratio is known as the power gain.
Final Words
As you can see, understanding hFE is critical to knowing how to use a BJT in a circuit, especially for amplification purposes. This ratio is also important in switching, biasing, and performance optimization circuits because it enables you to determine the appropriate base current to use to control the transistor. Once you know the required activation current, you can calculate the most suitable resistor to use at the base.
So whether you use PNP or NPN BJTs, we recommend calculating hFE using known resistances or a multimeter to determine the circuit’s design and component values.
