Positive SSL
A 70dB,21.9V
B 70dB,16.2V
C 60dB,16.2V
D 60dB,21.9V

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


A 0.775 V
B 7.775 V
C 77.5 V
D 3.875 V

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


A 98
B 115
C 144
D 200

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


A 100dB
B 60dB
C 40dB
D 20dB

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


A 8mV
B 4mV
C -3.2V
D 20μV

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


A 5W
B 10W
C 15W
D 20W

-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 10mV
B 0.1V
C 0.5V
D 0.25V

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


A 3dB
B 6dB
C 9dB
D 12dB

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