The frequency at which the amplifier gain decreases to
At this point, the output power becomes half of its mid-band value.
The upper cutoff frequency marks the upper limit of the useful frequency range of the amplifier.
3-dB Concept
The decrease in gain at the cutoff frequency.
At the frequency cutoff:
- The voltage gain drops to 70.7
\% of the mid-band gain. - Power drops to half of what it was in the middle.
Mathematically,
Therefore, the upper cutoff frequency is also known as the 3-dB frequency.
Factors Affecting Upper Cutoff Frequency
- Capacitances of internal transistors: Base-collector and base-emitter capacitances are examples of internal transistor capacitances that lower the high-frequency gain.
- The Miller Effect: Reduces the upper cutoff frequency by increasing the effective input capacitance.
- Capabilities of Parasites: The high-frequency response is further impacted by stray capacitances from circuit components and connections.
- Configuration of the Circuit: The higher cutoff frequency and the frequency response are affected by various amplifier topologies.
Relation with Capacitances
At high frequencies, capacitances play a major role in determining the amplifier behavior.
The upper cutoff frequency can be approximated as:
where
- R is the equivalent resistance seen by the capacitance
- C is the equivalent resistance seen by the capacitance
As capacitance increases, the upper cutoff frequency decreases.
Dominant Pole Concept
Multiple capacitances produce multiple high-frequency poles in practical amplifiers. Nonetheless, the frequency response is typically most affected by one pole.
The top cutoff frequency of the amplifier is mostly determined by this pole, which is referred to as the dominating pole.
Complex amplifier circuit analysis is made easier by the dominating pole.
Advantages
- Assists in Determining the Frequency Limit: Establishes the maximum frequency at which the amplifier can function efficiently.
- Beneficial for Amplifier Design: Helps engineers in creating circuits with the necessary bandwidth.
- Crucial for Applications with High Frequency: Gives information about how amplifiers operate at high frequencies.
Disadvantages
- Sensitive to the Effects of Parasites: The practical value may change due to wiring effects and stray capacitances.
- Depending on the Circuit Conditions: The circuit configuration and biasing affect the upper cutoff frequency.
- Restricts the Use of High Frequency: After this frequency, amplifier gain rapidly drops.
Applications
- Systems of Communication: Used to calculate amplifier bandwidth in RF and telecom circuits.
- Signal processing and audio: Helps in maintaining appropriate amplification within the necessary frequency range.
- Electronic Circuits with High Speed: Crucial for creating circuits that run at high frequencies.
- Design of Integrated Circuits: Used to examine transistor-based circuits frequency constraints.