Gain–bandwidth product of BC547 datasheet confusion

• Gain–bandwidth product of BC547 datasheet confusion
  Posted by petert on 16 Nov, 2020 20:36
• When you look at the datasheet from the BC547, the corresponding line reads:
  
  

  Quote

  f_T  Current Gain Bandwidth Product   VCE=5V, IC=10mA, f=100MHz   300MHz (Typical)

  
  Sorry if this is obvious, but what is now the GBWP (or f_T), 100MHz or 300MHz? Why are there two frequencies?
  
  Since the GBWP (or f_T) specifies the limit frequency that will still be amplified to unity gain, does it also mean I can switch frequencies up to f_T without degradation/distortion? If it only applies to amplification, how do I know the maximum frequency I can switch?
  
  
• #1 Reply
  Posted by TimFox on 16 Nov, 2020 20:43
• That data sheet line means that the parameter f_t is measured at 100 MHz.  I.e., the value of h_fe is guaranteed to be > 3 at 100 MHz.
• #2 Reply
  Posted by petert on 16 Nov, 2020 23:24
• Thanks.
  
  What about this part?
  
  

  Quote

  Since the GBWP (or fT) specifies the limit frequency that will still be amplified to unity gain, does it also mean I can switch frequencies up to fT without degradation/distortion? If it only applies to amplification, how do I know the maximum frequency I can switch?

  I mean switch, as in using a transistor as a switch.
• #3 Reply
  Posted by T3sl4co1l on 17 Nov, 2020 01:30
• Two problems:
  1. Switching is a large-signal process, so small-signal parameters (h_fe, f_T, etc.) aren't very meaningful.  It's not that they're inapplicable, more that it's a hand-waving relationship; large-signal parameters can be measured in a similar way, but a more robust method must be employed to describe the nonlinear response.  (Which isn't ever done with general purpose transistors, but is sometimes provided for RF power transistors -- the data are used in simulators that do harmonic balance and load pull calculations.)
  2. Switching is a wideband signal, and presumably you want it to look like it's switching, not just a pile of mush.  So, you'll typically be below f_T / 10 say.
  
  Or put another way, expect the edge rate to be comparable to f_T, or a modest fraction of it (so, 1/3rd or ~100MHz or 10ns, for example), so whatever flat top/bottom you want inbetween needs to be as many times longer than that.
  
  And that's still pretty respectable, getting say 30MHz from a BC547.  It won't look very sharp up there, but it can do it.  Don't be stingy with the bias current, expect to need a good 10-30mA (and 0.5-3mA on whatever's driving the base) to do it.
  
  I've ran 2N3904s (basically the same thing, but 200mA) and 2N4401s (600mA) about as fast, and at about as much bias; I haven't done BC547 specifically, but it should be in that range as well.
  
  Tim