# Common gate with low input impedance

This is from my textbook talking about the advantages of a common gate amplifier. The author says that the common-gate stage is usually superior to that of the common-source stage for two reasons.

However, what makes me confused is about the second reason. The low input impedance is only an advantage when the input source is a current signal.

So with low input impedance the common gate is only better than the common source when the input source is a current signal. When the input source is a voltage signal, the common source is better.

Do you agree with this? • The highlighted text doesn't say that the input has to be a current source. – Mario Sep 25 '16 at 17:21
• But if the input is not current signal then common gate is not better than common source for that reason. – anhnha Sep 25 '16 at 17:28
• For what reason? – Mario Sep 25 '16 at 17:31
• Since the Input to source is low Z, a Voltage source with low Zout is better and the result is more bandwidth or fastest slew rate than common source at the expense of no reduction of current. same for common base. i.e. faster switching, or more BW in linear mode, when needed. – Tony Stewart Sunnyskyguy EE75 Sep 25 '16 at 17:40

So with low input impedance the common gate only better than common source when the input source is current signal. When the input source is voltage signal, the common source is better.

Do you agree with this?

No, the input impedance is not zero and can easily be made (say) 100 ohms or 200 ohms by including a series resistor.

The main advantage of common-base (or common-gate) is that the internal miller capacitance between drain and gate (or collector-base for a BJT) has very little effect at high frequencies.

Normally for a common source or emitter, the miller cap between drain (collector) and gate (base) significantly truncates high frequency gain unless you are driving the gate (or base) with a really hard low impedance signal. Yes you have to do that for a common source (base) but the problem isn't as bad at HF.

The textbook says the common gate has advantages over the common source for the frequency response -- not the voltage gain -- so it is correct. It's not comparing the common source to the common gate for voltage signal applications specifically. Each circuit has its own advantages and disadvantages -- the common source has a better input impedance for a voltage input, but the common gate has better frequency response.

Despite its lower input impedance, the common gate is nonetheless useful in applications which use voltage signals. For example, the MOSFET cascode is a common source followed by a common gate which has an overall voltage gain comparable to the common source but much higher bandwidth.

A common source amplifier by itself works by converting the input voltage into a current (the drain current) using the MOSFET's transconductance $g_{\text{m}}$. The load resistor $R_{\text{L}}$ then converts the current back into a voltage which is taken as the output. Hence the voltage gain is approximately $-g_{\text{m}}R_{\text{L}}$. Unfortunately, the high voltage gain from gate to drain results in a significant Miller effect that reduces the bandwidth considerably.

The cascode reduces the Miller effect by connecting a common gate amplifier between the common source MOSFET's drain and the load resistor. The overall voltage gain of the cascode is basically unchanged: the common gate acts as a current buffer (since the two MOSFETs share the same drain current) so the same current flows through the load resistor as in the common source amplifier (thus producing the same output voltage). However, the lower input impedance of the common gate means that the voltage gain from the input to the drain of the common source MOSFET is significantly lower -- this also reduces the Miller effect and improves the bandwidth. Even though the input to the overall cascode amplifier is a voltage signal the common gate amplifier is useful.