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G8MNY > TECH 21.07.03 23:12l 79 Lines 2956 Bytes #999 (0) @ WW
BID : 6575_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: an AF amp stage
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Sent: 030721/1430Z @:GB7CIP.#32.GBR.EU #:6575 [Caterham] $:6575_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. 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.
/QSL
73 De John, G8MNY @ GB7CIP
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