The Early Effect

What is the Early effect?

The Early effect is a phenomenon in bipolar junction transistors (BJTs) where the collector current (Ic) increases with increasing collector-emitter voltage (Vce), even though the base current (Ib) is held constant. This is because the reverse bias between the collector and base regions causes the depletion region to widen, which in turn reduces the effective base width. This narrower base width allows more minority carriers to reach the collector, resulting in an increase in Ic.

How does the Early effect affect transistor performance?

The Early effect can have a significant impact on the performance of BJTs, particularly in high-voltage applications. It can cause distortion in amplifiers and can also reduce the output impedance of transistors. In some cases, the Early effect can even cause instability in circuits.  It limits the maximum gain possible in resistor loaded common emitter amplifiers by effectively acting as an additional load resistor between the collector and emitter.

How can the Early effect be mitigated?

There are a few ways to mitigate the Early effect:

• Use a transistor with a high Early voltage (VA). The Early voltage is the voltage at which the collector current extrapolates to zero. Transistors with a high Early voltage are less susceptible to the Early effect.

• Use a transistor with a narrow base width. A narrow base width reduces the effect of the widening depletion region on the effective base width.

• Use a feedback circuit to compensate for the Early effect. A feedback circuit can be used to counteract the change in Ic caused by the Early effect.

• Keep the collector voltage constant, for example by using the Cascode configuration.

The Early Effect in datasheets

The Early effect causes the Hfe value to increase with increasing collector-emitter voltage (Vce).

Here is a example of how the Early effect can be seen in a transistor Hfe chart:

| Vce | Hfe |

|---|---|

| 0.5 V | 100 |

| 1.0 V | 105 |

| 2.0 V | 110 |

| 5.0 V | 120 |

In this example, the Hfe value increases from 100 to 120 as the Vce increases from 0.5 V to 5.0 V. This increase is due to the Early effect.

An example is in the BC547 datasheet.

The Early effect in bipolar junction transistors (BJTs) causes the collector current (Ic) to increase with increasing collector-emitter voltage (Vce), even though the base current (Ib) is held constant. This increase in Ic can be modeled by adding an extra apparent resistance (RL') in parallel with the normal collector emitter conduction route that passes the extra current. The impact is exactly like connecting a resistor in parallel with the collector and emitter of a transistor with no Early Effect and it acts as an additional output load.

The value of RL' is given by:

RL' = 1 / (dIc/dVce)

where dIc/dVce is the rate of change of Ic with respect to Vce.

The Early voltage

The Early voltage is typically expressed in volts and can range from tens of volts to hundreds of volts, depending on the specific BJT. It is an important parameter for high-voltage applications, as it affects the output impedance and linearity of the transistor.

To measure the Early voltage directly, one can plot the collector current versus the collector-emitter voltage for a fixed base-emitter voltage. The Early voltage is then the point where the collector current extrapolates to zero.

Here's a summary of the key points about the Early voltage:

• It represents the voltage at which the transistor's internal resistance becomes infinitely high.

• It affects the output impedance and linearity of the transistor, particularly in high-voltage applications.

• It is typically measured in volts and can range from tens to hundreds of volts.