An AF amplifier stage
by John Stockley G8MNY
This simple amplifier circuit is easy for calculations.
To get 1/2 the 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.
Technically the amount that (G-1)x Rb component that affects the O/P Z
it will also depend the I/P source Z.
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
Intrinsically limited by the transistor's FT when the Hfe becomes 1, &
component layout (inter capacitance) causing Miller HF N.F.B. effects.
HF Compensation
HF loss can be compensated for by putting a suitable C across Re to give
+3dB boost were Xc=Re eg where the measure drop is -3dB, & the 6dB/Octave
lift after that should flatten the amp losses out. The input Z will be
reduced at HF though. Not often used!
Example
So in the above example Collector should be around +6V
Gain about 9
O/P Z about 900 ohms +XCout
I/P Z about 5K ohms +XCin
LF response with Input source Z of zero, & O/P load of 10K...
I/P -3dB LF roll off, @ 31Hz where Xc = 5Kohms
O/P -3dB LF roll off, @ 29Hz where Xc = 10.9Kohms
Giving -6dB @ 30Hz & 12dB/Octave LF cut.
73 De John, G8MNY @ GB7CIP
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