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G8MNY > TECH 04.01.03 14:56l 78 Lines 2955 Bytes #999 (0) @ WW
BID : 36471_GB7CIP
Read: GUEST OE7FMI
Subj: an AF amp stage
Path: DB0MRW<DB0RGB<OK0PPL<RZ6HXA<SP7MGD<GB7YKS<GB7PZT<GB7MAX<GB7COV<GB7COV<
GB7CIP
Sent: 030104/0844Z @:GB7CIP.#32.GBR.EU #:36471 [Caterham] $:36471_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
This simple amp circuit is easy for calculations.
+9V ---------------------------------------------------
!
Rc
!
-------! Cout
! !----!!----
Rb !
! !/
I/P ---!!---------!
Cin !\e
!
Re
!
0V-------------------------------------------------
To get 1/2 DC swing on the O/P then biasing Rb = Hfe x (Rc+Re)
This is because we want the same voltage C-E (almost the same as across
Rb) as across the total load R of Rc+Re.
Gain G approx = Rc/Re (Rc may be lower due to external load)
With high Hfe then Ie approx = Ic, so the emitter NFB Re controls the
collector current making the voltage gain just the voltage drop ratio of
Rc/Re. Assuming no external loads. And for high gain applications Re
includes the internal emitter R of the transistor (typically a few ohms).
O/P Z = XCout + (Rc // ((G-1) x Rb))
This is the added components, including the apparent fraction of the bias
Rb with load current in it.
"//" means in parallel, many of the paralleled terms are insignificant.
I/P Z = XCin + ((Hfe x Re) // (Rb/(G+1)))
This is the added components, including the apparent fraction of the bias
Rb with input current in it.
"//" means in parallel, many of the paralleled terms are insignificant.
LF Roll off
Cin & Cout affect the LF response. Basically each one will give -3dB &
6dB/Octave roll off when Xc equals the source + load Zs.
HF Response
Basically limited by the transistor's FT when the Hfe becomes 1, &
component layout (inter capacitance) causing Miller HF NFB effects.
Example
+12V -----------------------------------------------
!
1K
!
-------! + Cout
! !----!!---- Output
100K ! 0.5u !
+ ! !/ !
I/P ---!!---------! Hfe=100 10K Load
Cin !\e !
1u ! !
100 !
! !
0V-------------------------------------------------
So in the above example Collector should be around 6V
Gain about 9
O/P Z about 900R +XCout
I/P Z about 5K +XCin
LF response with Input source Z of zero, & O/P load of 10K...
I/P -3dB LF roll off, @ 31Hz Xc = 5K
O/P -3dB LF roll off, @ 29Hz Xc = 10.9K
Giving -6dB @ 30Hz & 12dB/Octave LF cut.
73 De John, G8MNY @ GB7CIP
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