Understanding Single Tuned Amplifier Frequency Response

In electronic amplifier circuits, the frequency response describes how the amplifier's gain and phase shift vary with the input signal's frequency. For a single tuned capacitance coupled amplifier, the goal is typically to amplify signals within a specific frequency band. This amplification is achieved using resonant circuits, often called "tuned circuits".

Role of Tuned Circuits

A tuned circuit typically consists of an inductor (L) and a capacitor (C). This combination resonates at a specific frequency, known as the resonant frequency ($f_0$). The formula for the resonant frequency is:

$$f_0 = \frac{1}{2\pi\sqrt{LC}}$$

At this resonant frequency, the circuit exhibits maximum impedance (for a parallel LC circuit) or minimum impedance (for a series LC circuit), which allows for maximum voltage gain. The amplifier's frequency response, particularly its peak gain and bandwidth, is heavily influenced by the characteristics of this tuned circuit, specifically its resonant frequency ($f_0$) and its quality factor (Q). The Q factor determines the sharpness of the resonance peak and is related to the circuit's resistance (R):

$$Q = \frac{\omega_0 L}{R} \quad \text{or} \quad Q = \frac{1}{\omega_0 CR}$$ where $\omega_0 = 2\pi f_0$.

Analyzing Input vs. Output Circuits

In a single tuned amplifier configuration:

• Output Circuit: The resonant (tuned) circuit is typically placed in the output stage (e.g., as the collector or drain load). This circuit is directly responsible for selecting the desired frequency band and providing the high gain at the resonant frequency. Changes in the components (L, C, R) of the output tuned circuit directly alter the resonant frequency and the Q factor, thereby defining the amplifier's peak frequency, bandwidth, and overall shape of the frequency response curve.
• Input Circuit: The input circuit includes the coupling capacitor and the input impedance of the amplifier stage. While the coupling capacitor affects the low-frequency response by forming a high-pass filter with the source impedance and the input impedance, it doesn't determine the central peak or the high-frequency roll-off characteristic associated with the *tuning* element. The primary frequency-selective behavior comes from the resonant circuit.

Conclusion on Frequency Response Dependence

Given that the defining characteristic of a tuned amplifier is its selective amplification around a resonant frequency, and this selectivity is primarily engineered using a resonant circuit (LC tank), the frequency response is fundamentally dependent on the parameters of that resonant circuit. In a single tuned configuration, this circuit is predominantly located in the output stage. Therefore, the frequency response relies heavily, and in the context of the *tuning* aspect, solely on the output circuit.

Answer Choice Evaluation

Based on the analysis:

• Option 1 (Input circuit only): Incorrect, as the output tuned circuit is the primary frequency-determining element.
• Option 2 (Output circuit only): Correct, as the output tuned circuit defines the resonant frequency and bandwidth characteristics central to the amplifier's frequency response.
• Option 3 (Both circuits): While the input circuit affects overall signal transfer and low-frequency cutoff, the *tuned* frequency response characteristic is dominated by the output circuit.
• Option 4 (Neither circuit): Incorrect, as the circuit components inherently determine the response.

The correct option is that the frequency response depends only on the output circuit.