Voltage feedback and current feedback.

In summary: Shannon's seminal work on communication theory demonstrated that any two devices that can exchange information - be they bits or symbols - will inevitably form a communication channel. In practical terms, this means that the output of one device can be used as the input to another, without the need for a physical connection between the devices. This is accomplished by using a device known as a transducer, which takes the output of one device and converts it into an input for another. In the case of an amplifier, the transducer is the output transistor. In summary, the gain of an amplifier is something like \frac{\mu R_{l}}{R_{p} + R_{k}(\mu +1) + R
  • #1
bitrex
193
0
Looking at the equations for the gain of a common cathode tube amplifier with the cathode resistor unbypassed (though the question in general could apply to similar configurations with BJTs and FETs) I see that the gain of the amplifier is something like [tex]\frac{\mu R_{l}}{R_{p} + R_{k}(\mu +1) + R_{l}}[/tex], where Rl is the load resistance, Rp is the dynamic plate resistance, mu is the amplification factor, and Rk is the unbypassed cathode resistance. This is degenerative feedback where the feedback is proportional to the current through the device - the gain of the amplifier is reduced and the dynamic plate resistance is increased. However, in a cathode follower, when the emitter resistor is unbypassed the dynamic plate resistance is decreased by the factor [tex]\frac{R_{p}}{\mu +1}}[/tex]. This must be voltage feedback since the input impedance is increased and the output impedance is reduced. What I can't see is how the feedback changed from current feedback to voltage feedback by just taking the output from the cathode instead of the anode.
 
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  • #2
It's current feedback, cfb, because the error signal is a current. In a voltage feedback loop, vfb, the error signal is a voltage.

With vfb, the output voltage (or current with a sense resistor) is fed back through a resistive divider, to the base of the input transistor which is driven until the differential voltage across the 2 bases (input is a differential pair) is reduced to virtual zero.

With cfb, the output voltage (or current with a sense resistor) is fed back through a resistive divider, to the common emitter of a totem pole emitter follower. The upper emitter sources current and the lower emitter sinks current. The output drives the feedback current to virtual zero when the loop is closed.

It's all about the error signal. The terms cfb & vfb refer to the error signal being a current or a voltage resp. Does this help?

Claude
 
  • #3


Voltage feedback and current feedback are two types of feedback systems commonly used in electronic circuits. In voltage feedback, the feedback signal is derived from the output voltage of the circuit, while in current feedback, the feedback signal is derived from the output current of the circuit.

In the context of a common cathode tube amplifier, the equations for gain show that the feedback in this configuration is degenerative, meaning it reduces the overall gain of the amplifier. This is because the feedback signal is proportional to the current through the device, which is determined by the unbypassed cathode resistance.

However, when the amplifier is configured as a cathode follower, the feedback signal is now derived from the output voltage at the cathode, which is determined by the unbypassed emitter resistor. This results in a decrease in dynamic plate resistance, which can be seen as voltage feedback.

The key difference between voltage and current feedback lies in the location of the feedback point. In voltage feedback, the feedback signal is taken from the output voltage, while in current feedback, the feedback signal is taken from the output current. The feedback mechanism itself remains the same, but the resulting effect on the circuit can be different.

In conclusion, taking the output from the cathode instead of the anode in a common cathode tube amplifier results in a change from current feedback to voltage feedback due to the change in the feedback point. Both types of feedback have their own advantages and disadvantages and are used in different circuit configurations depending on the desired outcome.
 

1. What is the difference between voltage feedback and current feedback?

Voltage feedback is a type of feedback in which the output voltage of a circuit is compared to a reference voltage and then amplified to produce a feedback signal. Current feedback, on the other hand, compares the output current of a circuit to a reference current and amplifies the difference. In voltage feedback, the output voltage is directly controlled, while in current feedback, the output current is controlled.

2. How does voltage feedback affect stability in a circuit?

Voltage feedback can affect stability in a circuit by increasing the gain and bandwidth of the circuit. This can result in higher frequency response and faster transient response. However, if the feedback loop is not properly designed, it can also lead to instability and oscillations in the circuit.

3. Which is better for high-speed applications, voltage feedback or current feedback?

For high-speed applications, current feedback is generally preferred over voltage feedback. This is because current feedback has a wider bandwidth and better stability, making it more suitable for high-frequency circuits. It also has a lower output impedance, which can improve performance in high-speed applications.

4. What are some common applications of voltage feedback and current feedback?

Voltage feedback is commonly used in operational amplifiers, audio amplifiers, and voltage regulators. Current feedback is often used in high-speed amplifiers, video circuits, and other high-frequency applications. Both types of feedback can also be used in feedback control systems, such as in motor control and power converters.

5. How do I choose between voltage feedback and current feedback for my circuit?

The choice between voltage feedback and current feedback depends on the specific requirements of your circuit. If you need high gain and stability, voltage feedback may be a better option. If you require a wider bandwidth and better performance at high frequencies, current feedback may be more suitable. It is important to carefully consider the specifications and design of your circuit to determine which type of feedback will be most effective.

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