When you have completed this exercise, you will be able to determine the frequency response of a an oscilloscope. Voltage gain (Av), the voltage ratio of the input signal to the output signal, can be expressed logarithmically as well as arithmetically. The unit for logarithmic gain is the decibel (db). The logarithmic voltage gain in decibels (dbv) is plotted against the input signal frequency to give a frequency response curve. You will use decibels to determine the frequency response and bandwidth of a two-stage transformer- The unit of logarithmic gain is the a. decibel. b. log. The logarithmic voltage gain in decibels is calculated from the arithmetic voltage gain. Voltage gain in dbv = 20 log 10 (V o i ) or 20 log 10 (Av) One reason decibels are used to express gain is that human hearing has a logarithmic response. If the decibel level of a loudspeaker output is doubled, the sound seems twice as loud to the human ear. Doubling the decibel level is equivalent to a tenfold increase in arithmetic gain. To the human ear, if the decibel level of a loudspeaker output is doubled, the sound seems a. the same. b. twice as loud. c. ten times as loud. FACET by Lab-Volt 273
Transformer Coupling ±2 db between 200 Hz and 10 khz. a. the RC coupling. b. transformer coupling. c. the NPN transistors. circuit for frequency measurements, as shown. 274 FACET by Lab-Volt
Measure V A with reference to ground. V A = Vdc (Recall Value 1) While observing the signal on oscilloscope channel 1, adjust the sine wave for a 1 khz, 100 mv pk-pk ac input signal (V i1 ) at the base of transistor Q1. Connect the channel 2 oscilloscope probe to the second-stage ac output signal (V o2 ) at the collector of Q2. FACET by Lab-Volt 275
Transformer Coupling Measure V o2 at a frequency of 1 khz. V o2 = mv pk-pk (Recall Value 2) Adjust the sine wave generator for 200 Hz and, if necessary, readjust the input signal (V i1 ) to 100 mv pk-pk. Measure V o2 at a frequency of 200 Hz. V o2 = mv pk-pk (Recall Value 3) Adjust the sine wave generator for 3 khz and, if necessary, readjust the input signal (V i1 ) to 100 mv pk-pk. Measure V o2 at a frequency of 3 khz. V o2 = mv pk-pk (Recall Value 4) Adjust the sine wave generator for 10 khz and, if necessary, readjust the input signal (V i1 ) to 100 mv pk-pk. Measure V o2 at a frequency of 10 khz. V o2 = mv pk-pk (Recall Value 5) Calculate the overall loaded circuit gain for an input frequency of 200 Hz (Avc (L) = V o2 i1 ). Avc (L) = (Recall Value 6) 200 Hz mv pk-pk (Step 6, Recall Value 3) 1 khz mv pk-pk (Step 5, Recall Value 2) 3 khz mv pk-pk (Step 7, Recall Value 4) 10 khz mv pk-pk (Step 8, Recall Value 5) 276 FACET by Lab-Volt
Calculate the overall loaded circuit gain for an input frequency of 1 khz (Avc (L) = V o2 i1 ). Avc (L) = (Recall Value 7) Calculate the overall loaded circuit gain for an input frequency of 3 khz (Avc (L) = V o2 i1 ). Avc (L) = (Recall Value 8) Calculate the overall loaded circuit gain for an input frequency of 10 khz (Avc (L) = V o2 i1 ). Avc (L) = (Recall Value 9) circuit. gains (Avc (L) ). The Y-axis shows your approximate calculated gains. FACET by Lab-Volt 277
Transformer Coupling The X-axis of the frequency response curve is a logarithmic scale for the range of input signal frequencies (100 Hz to 200 khz). Do the gains on the frequency curve increase sharply before a frequency of 1 khz and decrease sharply after 100 khz? a. yes b. no Are the gains for frequencies in the frequency range of 1 khz to 20 khz generally within the a. yes b. no Voltage gain can be expressed in logarithmic units called decibels; decibels are plotted against input signal frequency for a frequency response curve. 278 FACET by Lab-Volt
1. Expressing gain in decibels is desirable because human hearing is a(n) a. arithmetic response. b. geometric response. c. logarithmic response. d. exponential response. 2. The logarithmic voltage gain equals a. 10 log 10 (Av). b. 20 log 10 (Av). c. 20 x Av. d. 10 ). 3. frequencies a. above 100 khz. b. less than 20 Hz. c. between 1 khz and 10 khz. d. between 100 Hz and 100 khz. 4. because a. b. c. the transformer steps down the ac signal. d. of transformer impedance matching. 5. a. high current gains. b. low power consumption. c. high voltage gains. d. no signal distortion. FACET by Lab-Volt 279