A **current-feedback amplifier (CFA)** achieves a high slew rate through its distinct internal architecture, which decouples the slew rate from the closed-loop gain of the amplifier, unlike a voltage-feedback amplifier (VFA). Here's how this works in detail:
### 1. **Current Feedback Path:**
- In a CFA, the input stage consists of a low-impedance inverting input, which allows current to flow directly into the amplifier's feedback network.
- This direct current feedback mechanism minimizes the dependence of the amplifier's performance on its internal compensation capacitances.
### 2. **Low Impedance at the Inverting Input:**
- The inverting input of the CFA has a very low impedance, which means it can respond to changes in input voltage more quickly, as there is less voltage drop across the input stage.
- The voltage changes at the output are driven by the current through this low-impedance input, and the speed of response is primarily determined by the current, not the voltage.
### 3. **High Bandwidth Transimpedance Stage:**
- After the input stage, the CFA has a transimpedance stage (voltage-to-current conversion), which processes the current and translates it to the output.
- This transimpedance configuration responds quickly to the current changes at the input, further boosting the speed of operation.
### 4. **Slew Rate Independence from Gain:**
- In a VFA, the slew rate is typically determined by the internal compensation capacitor and the current available to charge or discharge it. The compensation mechanism in VFAs makes their slew rate sensitive to the gain-bandwidth product.
- In contrast, the slew rate of a CFA is not constrained by this gain-bandwidth relationship. The current feedback architecture allows the output voltage to change quickly in response to input current, leading to a higher slew rate.
### 5. **Drive High Capacitance Loads Efficiently:**
- CFAs can drive capacitive loads more effectively than VFAs. This capability is crucial in high-speed applications where capacitive loading could otherwise limit the performance of an amplifier.
### Summary:
A current-feedback amplifier achieves high slew rates primarily because the feedback is based on current rather than voltage, allowing it to respond more quickly to input signals. The low impedance of the inverting input and the current-controlled feedback loop decouple the slew rate from the closed-loop gain, making it much faster compared to traditional voltage-feedback amplifiers. This makes CFAs particularly suitable for high-speed and wideband applications.