Hi,
i'm just reading a chapter on basic power amplifiers, and it starts with the class A amplfier. I'm a bit confused by how they're defining output power and efficiency and was wondering whether someone could clarify it for me?
They start by writing the power gain as \(A_p=A_vA_i\) where \(A_v=V_c/V_b\), \(A_i=I_c/I_s\) and \(I_c\) is the ac current through the parallel combination \(R_c=R_C||R_L\). Then later in the chapter they define the power gain as "the ratio of output power (power delivered to the load) to the input power":
\(A_p = P_L/P_{in} = \frac{(V_L)^2/R_L}{(V_{in})^2/R_{in}} = (A_v)^2(\frac{R_{in}}{R_L})\).
Am I right in thinking that these are two different versions of power gain and are not equivalent? The first one takes into account the power dissipated by \(R_C\) while the second one does not?
That leads me to the output power. Above they defined output power as "power delivered to the load", but in the next section they write it as \(P_{out}=V_{c(max)}I_{c(max)}\) which, for the case where the Q-point is at the center of the ac load line, will be equal to \(P_{out}=(0.707V_{CEQ})(0.707I_{CQ})=0.5I_{CQ}V_{CEQ}\). But this second one takes into account the power dissipated by \(R_C\), so it is not just "the power delivered to the load"?
Finally, they use this last defintion of output power to obtain the maximum effciency (when the Q-point is at the center of the ac load line) as
\(n_{max}=P_{out}/P_{DC} = \frac{0.5I_{CQ}V_{CEQ}}{I_{CC}V_{CC}} = \frac{0.5I_{CQ}V_{CEQ}}{I_{CQ}*2V_{CEQ}} = 0.25\).
But this seems a strange way to define efficiency, since you are including the power dissipated by \(R_C\) and not just the power dissipated by the load \(R_L\) (i.e. the thing you actually want to power!). Why do it like this?
Thanks for any help!
i'm just reading a chapter on basic power amplifiers, and it starts with the class A amplfier. I'm a bit confused by how they're defining output power and efficiency and was wondering whether someone could clarify it for me?
They start by writing the power gain as \(A_p=A_vA_i\) where \(A_v=V_c/V_b\), \(A_i=I_c/I_s\) and \(I_c\) is the ac current through the parallel combination \(R_c=R_C||R_L\). Then later in the chapter they define the power gain as "the ratio of output power (power delivered to the load) to the input power":
\(A_p = P_L/P_{in} = \frac{(V_L)^2/R_L}{(V_{in})^2/R_{in}} = (A_v)^2(\frac{R_{in}}{R_L})\).
Am I right in thinking that these are two different versions of power gain and are not equivalent? The first one takes into account the power dissipated by \(R_C\) while the second one does not?
That leads me to the output power. Above they defined output power as "power delivered to the load", but in the next section they write it as \(P_{out}=V_{c(max)}I_{c(max)}\) which, for the case where the Q-point is at the center of the ac load line, will be equal to \(P_{out}=(0.707V_{CEQ})(0.707I_{CQ})=0.5I_{CQ}V_{CEQ}\). But this second one takes into account the power dissipated by \(R_C\), so it is not just "the power delivered to the load"?
Finally, they use this last defintion of output power to obtain the maximum effciency (when the Q-point is at the center of the ac load line) as
\(n_{max}=P_{out}/P_{DC} = \frac{0.5I_{CQ}V_{CEQ}}{I_{CC}V_{CC}} = \frac{0.5I_{CQ}V_{CEQ}}{I_{CQ}*2V_{CEQ}} = 0.25\).
But this seems a strange way to define efficiency, since you are including the power dissipated by \(R_C\) and not just the power dissipated by the load \(R_L\) (i.e. the thing you actually want to power!). Why do it like this?
Thanks for any help!
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