# maximum closed loop gain achievable with op-amp 741 etc.

#### PG1995

Joined Apr 15, 2011
832
Hi

Regards
PG

#### Papabravo

Joined Feb 24, 2006
19,588
It is just linear interpolation, but on a log-log scale. The plot is Gain in dB versus the logarithm of the frequency. Gain in dB is inherently logarithmic. 20 dB per decade is the slope of the frequency response plot.

• PG1995

#### PG1995

Joined Apr 15, 2011
832
Thank you, papabravo. I understand it now.

This Wikipedia article was quite useful, especially where it says:
To determine the number of decades between two frequencies, use the logarithm of the ratio of the two values: In the question in my last post, 20dB/decade is the slope and factor "log(5 kHz/1 kHz)" gives the number of decades between 1 kHz and 5 kHz frequencies.

Regards
PG

#### steveb

Joined Jul 3, 2008
2,436
PG,

It is good to memorize some key log scale conversions and keep them in your head. For example. On a power scale 3 dB is just about a factor of 2, and on a voltage/current scale, 6 dB is a factor of 2. Similarly, 10 dB is a factor of 10 on power scale and 20 dB is factor of 10 on voltage/current scale. With just these 2 facts memorized, you can do a lot of stuff in your head.

For example, here 5 kHz is one half of 10 kHz, so the number at 5 kHz is 6 dB higher than the 40 dB number at 10 kHz.

Over time, you can add other numbers. For example, knowing what 1 dB does can allow you to fine tune some estimates. Try it. It's not as hard as it may sound and it makes you look real smart when you can rattle stuff off instantly.

• PG1995