fT & GBW Are the Same

When we want to amplify higher frequencies, our amplifiers always run into declining gain at higher frequency. Both graphs following show this. The blue op amp has a gain of 1300 up to 1000Hz, then it trails off at a rate of 1/10 the gain for every decade of frequency.

The orange op amp has a gain of 39 up to 20kHz, then it also trails off. The surprising thing about op amps is that, once they trail off, they follow the same gain curve. You can buy an op amp that keeps up gain to somewhat higher frequency, but it will tend to oscillate.

Mathematically, this trailing off of gain shows the gain bandwidth product, GBW. On the slopy part, gain x freq = about 1.2MHz. This is a specification on the spec sheet of the op amp. Op amps just don't have much application above audio frequencies, so we have to turn to transistors.

The graph below seems to show the same curve, and in fact the trail off is the same as for op amps, 1/10 the gain for each decade of frequency. But transistors have a much higher GBW than op amps. The graph is for a medium-power transistor and the GBW is about 40 MHz. The common, $.05 2N3904 has GBW of 200MHz! Transistors are readily available with GBW of 1.1GHz, and for $.35 you get 9GHz, though the current and voltage ability are trailing off.

For transistors, GBW is called f_T.

Transistors can amplify at radio frequencies, and integrated circuits have transistors that amplify at radio frequencies. In fact, microprocessors from Intel and AMD have transistors that can go at 3GHz. If you want to make an RF amplifier, that is a special-knowledge field that requires $20,000 equipment like spectrum analyzer. When home circuit builders try to go above 100MHz, they generally get oscillations.