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
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
dBm 是以 1mW為參考基準
20 = `10 log (P/(1m))`
2 = `log (P/(1m))`
100 = `(P/(1m))`
P = 100mW
第一級 電壓增益 20dB，所以電壓放大 10倍。第二級 電壓增益 40dB，所以電壓放大 100倍。 第二級電壓輸出 1μ *10 *100 = 1mV
`P = V^2/R`
100m = `V^2/(1k)`
`V^2` = 100
V = 10 V
第總電壓增益 = `20log (V_o/V_i)`
= `20 log(100/(1μ))`
= `20 log(10^7)`
= `140 dB
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
Pi = `100^2 /(1k)`
= `10000 /(1k)`
= 10
Po = `30^2 /9`
= `900 /9`
= 100
Ap(dB) = `10 log (100/10)`
= `10 log 10`
= 10 dB
輸出訊號 電壓= 10m*100 = 1V
輸出雜訊 電壓= 0.1m*100 = 10mV (雜訊也被放大了)
S/N(dB)= `20 log(1/(10m))`
= 20 log 100
= 40dB
Ap=1000*100 = `10^5`
Ap(dB)=`10 log (10^5)`
= 50dB
`A_(V1)(dB) = 20dB`
`A_(V1) = 10`
`A_(V2)(dB) = 40dB`
`A_(V2) = 100`
`A_(VT) = 10*100` =1000
Vo = 1000*1μ =1mV