Passband gain

msddw

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P359 of the fourth edition of Tong Shibai's Model Electronics says: The ratio of the output voltage to the input voltage when the frequency is 0 is the passband gain, but how come (7.3.35) and (7.3.36) on P370 are the passband gain obtained when f=f0?

maychang

Strictly speaking, the sentence "Aup is the passband gain" in the previous red underline is a bit problematic.

The gain is not a constant, but varies with frequency. Even within the passband, the gain is not a constant. This is clearly seen in the curve in Figure 7.3.2. As the frequency changes from low to high, the gain begins to decrease when the frequency approaches the passband cutoff frequency fp (about one tenth of fp), but before one tenth of fp, the gain is basically a constant.

msddw

So does that mean that the gain when the complex part of the circuit's transfer function is equal to 1 is the passband gain?

maychang

That's not what I meant.

The red underlined part in the front and back of the figure on the second floor are contradictory. The front defines the ratio of the two voltages " in the passband " as the passband gain, and the back defines the ratio of the two voltages " when the frequency is zero ".

The problem with the first sentence is that "even within the passband, the amplification factor is not a constant", but the second sentence does not have this problem.

Therefore, it is appropriate to use the latter definition.

maychang

msddw posted on 2019-6-11 11:41 So is the gain when the complex part of the transfer function of the circuit equal to 1 the passband gain?

Is the gain when the complex part of the circuit's transfer function is equal to 1 the passband gain?

In fact, this definition is consistent with the red underlined definition in the first post. And this definition is applicable to high-pass, band-pass, etc.

msddw

In the fourth edition of Tong Shi Bai Model Electronics, P370, when f=0, the equation (7.3.35) is not equal to 0 as a whole?

maychang

When f=0, the first term in the brackets above the denominator of formula (7.3.35) is zero, and the second term is scattered.

Note: This is a bandpass filter circuit. The transfer function is completely different from the previous low-pass filter and cannot be applied.

maychang

msddw posted on 2019-6-11 18:23 In the formula (7.3.35) of the fourth edition of Tongshi Baimodian, P370, when f=0, is the whole not equal to 0?

From the previous post, when discussing frequency characteristics, there is no "zero" frequency, that is, there is no direct current. The so-called direct current is just the frequency tending to zero.

maychang

msddw posted on 2019-6-11 18:23 In the formula (7.3.35) of the fourth edition of Tongshi Baimodian, P370, when f=0, is the whole not equal to 0?

So I said on the 5th floor that your definition on the 3rd floor is applicable to high-pass, low-pass, band-pass..., which means that the characteristic of "passband amplification factor" is that the complex part is zero.

msddw

maychang posted on 2019-6-11 20:10 When f=0, the first term in the bracket above the denominator of formula (7.3.35) is zero, and the second term is scattered. Note: This is a bandpass filter circuit, and the transfer function...

The latter term diverges, so isn't the complex part infinite?

maychang

msddw posted on 2019-6-11 22:04 The latter term diverges, so isn't the complex part infinite?

"The latter term diverges, so isn't the complex part infinite?"

So f cannot be equal to zero.

Zero frequency does not exist.

The frequency can only be brought to zero.

maychang

msddw posted on 2019-6-11 22:04 The latter term diverges, so isn't the complex part infinite?

Bode plots often use logarithmic coordinates, in which "zero" is at infinity on the left and is impossible to reach.

人去搂空

msddw

maychang posted on 2019-6-11 22:29 "The latter term diverges, so isn't the complex part infinite?" So f cannot be equal to zero. Zero frequency does not exist...

Got it, thank you teacher

进击的程序员

Please advise, on page 369 of Tong Shi Bai, Auf=1+Rf/R(7.3.33) is this wrong?