The Early Effect And Vbc – An Often Overlooked Relationship
The early effect in bipolar junction transistors (BJTs) refers to the variation in the base-emitter voltage (Vbe) required to achieve a given collector current as the collector-base voltage (Vcb) changes. This occurs because the width of the depletion region in the base-collector junction is dependent on Vcb. As Vcb increases, the depletion width increases, effectively reducing the base width and increasing the likelihood that minority carriers will reach the collector without recombining.
What is Early Effect
In more detail, the early effect relates to how the electric field between the collector and base terminals in a BJT impacts the movement of charge carriers across the base region. This electric field can aid or oppose the movement of electrons and holes through the base, affecting the concentration of carriers that reach the collector and therefore impacting the collector current (Ic).
The strength of the early effect is quantified by the early voltage (VA), an intrinsic property of the specific BJT. A lower VA corresponds to a stronger early effect. VA is defined as the change in Vcb required to change Ic by a factor of e (approx 2.718) when the emitter current is held constant.
Relationship Between Early Effect and Vbc
Effect of Vbc on Current Flow
As mentioned above, Vcb directly impacts the width of the depletion region between the collector and base. A higher Vcb corresponds to a wider depletion region. This narrower effective base width allows more majority carriers (electrons in NPN transistors) injected from the emitter to reach the collector without recombining. Therefore, a given base current provides a proportionally larger collector current for higher Vcb.
Effect on Switching Speed
The modulation of the base width also impacts the switching speed of the transistor. The lower the Vcb, the larger the base width. This larger base storage increases the transistor’s turn-off time. As Vcb increases, the base width decreases, allowing the transistor to turn off faster. Therefore, the early effect causes the switching speed to improve at higher Vcb.
Optimizing Vbc for Faster Switching
To optimize a bipolar junction transistor for fast switching speeds, the collector-base voltage should be set higher than the minimum required for saturation. However, excessive Vcb will increase power dissipation during switch-on and switch-off transients. Therefore, choosing an appropriate Vcb involves balancing faster switching against energy loss during switching.
Example Circuits Showing Early Effect
BJT Common Emitter Amplifier
The early effect manifests clearly in common emitter amplifiers constructed using BJTs. In these amplifiers, Vcb changes along with the input voltage signal while the emitter current is approximately fixed by negative feedback. As Vcb changes over the cycle, the transistor’s current gain and other parameters are modulated due to the early effect. This leads to nonlinear distortion at high signal swings if left unaddressed.
Practical Considerations
To mitigate the early effect in common emitter BJT amplifiers, negative voltage feedback is employed from collector to base. This keeps Vcb relatively constant over the input signal cycle. However, too much feedback reduces voltage gain. So practical amplifiers keep Vcb changes small rather than eliminating them entirely.
Mitigating Early Effect
Using Feedback
As described above negative feedback (NFB) from collector to base in common emitter amplifiers is used to minimize changes in Vcb. The benefits of NFB come at the cost of reduced voltage gain. So a compromise has to be struck depending on the acceptable distortion limits.
Another approach is to use negative feedback purely within the emitter circuit rather than from collector to base. This emitter degeneration can improve linearity without sacrificing as much gain compared to NFB. However, high levels of emitter degeneration can limit bandwidth in high frequency and fast switching amplifiers.
Adjusting Biasing
In addition to feedback techniques, adjusting the bias point of the BJT also allows compensation for the early effect. Specifically, setting the nominal Vcb higher moves operation into a range where variations in Vcb have less impact on Ic. This reduces early effect induced distortion. However, higher Vcb biasing increases power consumption. So yet again, design tradeoffs must be made.
Conclusion
In summary, the early effect describes changes in current gain and other parameters of BJTs as Vcb changes due to its impact on base width. This effect becomes significant in amplifier circuits and other configurations where Vcb intentionally varies as part of normal operation.
Recognizing the underlying physics of the early effect allows engineers to purposefully design feedback and bias networks into circuits using BJTs. These compensate for early effect nonlinearities. However, the application requirements and constraints determine just how much early effect can be mitigated versus tolerated.
Through insightful circuit analysis and intentionally leveraging compensation techniques, bipolar junction transistor circuits can deliver excellent analog performance. This allows them to continue serving as key components in high fidelity amplification and signal conditioning applications despite inherent early effect sensitivity.
