揚聲器兩端電壓:
`P=V^2/R`
30 = `V^2/16`
480 = `V^2`
V = `sqrt(480)`
21.9V
麥克風功率:
P = (0.03)^2 / 300`
= 3μW
`P(dB)` = `10 log (30 / (3μ))`
= `10 log (10^7)`
= 70dB
用三用電表 dB 測量，其實是 dBm
0dBm 是指 600Ω 電阻功率為 1mW、此時電阻兩端電壓値是 0.775V。
功率換算成 dB值
`P(dB)` = `10 log A_p`
= `10 log (P/(1m))`
由題目可知，電表測得 20dB所以
20 = `10 log (P/(1m))`
2 = ` log (P/(1m))`
10^2 = ` P/(1m)`
P = 100mW
`P=V^2/R`
100m = `V^2/600`
100m*600 = `V^2`
V = `sqrt(60)`
= 7.75V
有輸入電阻及輸出電阻時，有負載效應，會影響放大倍率
`V_i` 到真正的`V_(i1)` 會有衰減，衰減比例如下:
`V_(i1) = V_i*(90k)/(10k+90k)`
`V_ (o1)` 到第二級輸入`V_(i2)` 也會有衰減，衰減比例如下:
`V_(i2) = V_(o1)*40/(10+40)`
`V_ (o2)` 輸出到負載 `V_o` 也會有衰減，衰減比例如下:
`V_o = V_(o2)*4/(1+4)`
整個放大器的過程：衰減 => 放大 => 衰減 => 放大 => 衰減
`V_o / V_i`
= `(90k)/(10k+90k)*10 * 40/(10+40) * 20 * 4/(1+4)`
` = 9/10*10 * 4/5 * 20 * 4/5`
= 115.2
Ap(dB)=`10 log ((P_o)/(P_i))`
=`10 log ((V_o^2/R_o)/(V_i^2/R_i))`
=`10 log (V_o^2/V_i^2* R_i/R_o)`
=`10 log (V_o/V_i)^2+10 log (R_i/R_o)`
=`20 log (V_o/V_i)+10 log (R_i/R_o)`
=`20 log 1000+10 log ((100k)/10)`
= 60dB + `10 log 10k`
= 60dB + 40dB
= 100dB
第一級與第三級先合併共 3dB + 37dB = 40dB
`A_V(dB)` = 40dB
40 = `20 log A_V`
2 = `log A_V`
`A_V` = 100
所以三級串聯電壓放大倍率共 100 * -20 = -2000
Vo = 2μ * -2000 = 4mV
-3dB點即半功率點，所以在20Hz和15kHz時的輸出功率為中頻段（1kHz）的一半
20/2=10W
`A_(V1)(dB) = 20dB`
`A_(V1) = 10`
`A_(V2)(dB) = 40dB`
`A_(V2) = 100`
`A_(VT) = 10*100` =1000
Vo = 1000*1μ =1mV
dBm 是以 1mW為參考基準
輸出為10 dBm，所以輸出功率
10 = `10 log (P_o/(1m))`
1 = ` log (P_o/(1m))`
10 = `(P_o/(1m))`
`P_o` = 10mW
`P = V^2/R`
10m = `V^2/25`
`V^2`=0.25
`V`=0.5 V
喇叭發出的音量與送至喇叭的功率成正比
P(dB)=`10log(4/1)`
= 10 log 4
=10 log(2^2)
=20 log 2
=6 dB